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Debugging practice

Bug hunts: debugging challenges on real code

Trace the failure, find the root cause, ship the fix.

bugs in the catalogue bugs fixed remain

Frontend

UI state, async races, validation

Backend

APIs, auth, incidents that made headlines

Database

Queries, consistency, caching

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Bug hunts: debugging challenges on real code

Fix more than 300 real, broken projects in JavaScript, Python and Java: a bug report, the code as shipped, and hidden tests that judge your fix. Free to read.

What is a bug hunt on CodeKairo?

A bug hunt is a debugging exercise on real code. You get a small project as it shipped — in JavaScript, Python or Java — and a bug report that says what is wrong, not where. You read the code, find the fault and fix it in the editor; hidden tests decide whether the fix is complete, and a partial fix shows as a partial pass count. There are more than 300 hunts, tagged by category (frontend, backend, database) and difficulty. A complete fix pays 50 XP and keeps your streak alive. Any hunt can be read without an account. From 1 January 2027, the Free plan allows one hunt a day.

Browse by language

JavaScript 190 hunts · 53 easy · 97 medium · 40 hard
Python 66 hunts · 20 easy · 37 medium · 9 hard
Java 55 hunts · 15 easy · 26 medium · 14 hard

Browse by layer

Frontend 44 hunts
Backend 216 hunts
Database 51 hunts

Every bug hunt

JavaScript (190)

Average Watch Time: Infinity Easy · Frontend
Back to 1970 Easy · Backend
Commission on the Tax Easy · Backend
Distance to the Decimal Easy · Backend
Divide by Zero, Dead in the Water Easy · Backend
Divided by the Sum, Not the Average Easy · Backend
Double-Charged at Checkout Easy · Frontend
Every Region in One Flight Easy · Backend
Four Years and a Day Off Easy · Frontend
Goto Fail, Goto Fail Easy · Backend
IN () Means Nothing Easy · Database
Infinite Scroll, Infinite Repeats Easy · Frontend
Left-Pad the World Easy · Backend
Posting on Anyone's Timeline Easy · Backend
Public by Default Easy · Frontend
Refunds for Money Never Taken Easy · Database
rm -rf on an Empty Variable Easy · Backend
Secrets for Every Fork Easy · Backend
Ten Seconds to Next Episode Easy · Frontend
The 130% Progress Bar Easy · Frontend
The 140-Character Lie Easy · Frontend
The Backup That Never Took Over Easy · Backend
The Cache Key With a Twist Easy · Database
The Checkout Meltdown Easy · Frontend
The Dividend Paid in Ghost Shares Easy · Database
The Error Code That Meant Yes Easy · Backend
The ETA That Went Negative Easy · Frontend
The Infinite Spinner Easy · Frontend
The Invisible Tweet Button Easy · Frontend
The Invite Link That Shared a Hash Easy · Backend
The Key That Joined the Wrong Repository Easy · Backend
The Lookup That Confirmed Every Email Easy · Backend
The Meters That Thought It Was 1920 Easy · Backend
The Move That Quartered Crits Easy · Frontend
The Negative-Price Flash Sale Easy · Backend
The Null Price Latte Easy · Backend
The One-Night Overcharge Easy · Frontend
The Password in the Debug Log Easy · Backend
The Plague the Pets Carried Easy · Backend
The Preview That Minted Someone Else's Token Easy · Backend
The Quote That Escaped the Link Easy · Frontend
The Regex That Trusted Email Easy · Backend
The Resurfaced Memory Easy · Frontend
The Slash That Flagged the Whole Web Easy · Backend
The Spreadsheet That Ran Code Easy · Backend
The Sure Hit That Missed One Time in 256 Easy · Frontend
The Tweet ID JavaScript Rounded Off Easy · Frontend
The Unauthenticated Customer Lookup Easy · Backend
The Upvote That Counted Twice Easy · Frontend
The Vanishing Search Results Easy · Frontend
Undo Send Never Works Easy · Frontend
Version 1.10 Beats Version 1.9 Easy · Backend
Your Password, in the Address Bar Easy · Backend
610,000 Shares at One Yen Hard · Backend
Both Halves Thought They Were Primary Hard · Database
Cache Stampede at Dawn Hard · Database
Drill or Live, Same Prompt Hard · Frontend
Everyone Tried to Fix It at Once Hard · Backend
Ghost in the Hostname Buffer Hard · Backend
Heartbleed's Little Cousin Hard · Backend
Keyset Without a Tiebreak Hard · Database
One Shard Took Every New User Hard · Database
Rate Limiter Free-for-All Hard · Backend
Root, No Password, Second Try Hard · Backend
Signed In as Somebody Else Hard · Backend
Six Degrees, Give or Take Hard · Frontend
The /24 That Swallowed YouTube Hard · Backend
The Alarm Queue That Went Quiet Hard · Backend
The Extra Second That Pegged Every CPU Hard · Backend
The Fetch That Reached the Metadata Service Hard · Backend
The Flag That Fired Old Code Hard · Backend
The Freelist That Ate the Raft Log Hard · Backend
The Gift Card Paid Twice Hard · Database
The Heatmap That Mapped the Base Hard · Backend
The Index That Started at Zero Hard · Backend
The Intermediate Nobody Was Watching Hard · Backend
The Late Enactor and the Empty Record Hard · Backend
The Opening Cross That Never Closed Hard · Backend
The Password Reset Sent to a Second Address Hard · Backend
The Player That Wore Out Disks Hard · Database
The Re-Mirror Storm Hard · Backend
The Reject Term Moved to the Top Hard · Backend
The Reply Meant for the Last User Hard · Backend
The Rover That Could Not Finish Booting Hard · Backend
The Scheduler That Never Converged Hard · Backend
The Seventy-Two Byte Password Hard · Backend
The Task That Ran Out of Stack Hard · Backend
The Transaction That Changed Its Own Name Hard · Backend
The Wallet Library That Killed Itself Hard · Backend
The Zero Root Everyone Trusted Hard · Backend
Twenty Thousand Spaces on the Home Page Hard · Backend
Two Spikes, 1.2 Seconds Apart Hard · Backend
Withdraw, Then Withdraw Again Hard · Backend
184 Billion Coins From Nowhere Medium · Backend
Access Denied at Midnight Medium · Database
Any Email You Asked For Medium · Backend
Anyone Could Initialise the Wallet Medium · Backend
Audio Before the Call Was Answered Medium · Backend
Crossing the Date Line Medium · Backend
Cursor Chaos After the Emoji Medium · Frontend
Document 000000001, No Login Required Medium · Backend
Entropy Removed to Silence a Warning Medium · Backend
Every Post, One Id at a Time Medium · Backend
False Positives at the Signup Gate Medium · Frontend
Ghost Rows in the Audit Log Medium · Backend
Half the Servers Had the Old Config Medium · Backend
Happy New Year, Dropped Messages Medium · Backend
Java, Newline, Script Medium · Frontend
Listening Everywhere, Asking Nothing Medium · Database
More Routes Than the Router Could Hold Medium · Backend
Negative Altitude, Underground Medium · Backend
Nine Digits and You're In the Meeting Medium · Backend
One Customer's Config Took Down the Edge Medium · Backend
One Domain Checked Three Times Medium · Backend
One Flipped Bit, Gossiped Everywhere Medium · Backend
One Plus Two Plus Three Is Twenty-Four Medium · Frontend
One Sensor, No Second Opinion Medium · Backend
One Typo, Half the Internet Medium · Backend
Readable Passwords at Rest Medium · Database
Release the Connection! Medium · Database
Retry After What? Medium · Backend
Scaled Down Overnight, Trampled at Dawn Medium · Backend
Scaling Up Past the Thread Limit Medium · Backend
Schrödinger's Follower Count Medium · Backend
Seven Minutes to Parse a Menu Medium · Backend
Shuffle Isn't Random Medium · Frontend
Sold Out, Sold Again Medium · Database
Somebody Else's Videos in My Takeout Medium · Database
Someone Else's Account Page, From Cache Medium · Backend
Someone Else's Bank Account Medium · Database
Stuck on Mute Medium · Frontend
The API That Returned More Than the Profile Medium · Backend
The Backup That Deleted the Primary Medium · Database
The Canary That Failed and Was Overruled Medium · Backend
The Car That Only Asked for Its VIN Medium · Backend
The Check That Hadn't Cleared Medium · Backend
The Cleanup That Deleted the Customers Medium · Database
The Config Value That Crashed Every App Medium · Frontend
The Console That Thought 2010 Was a Leap Year Medium · Backend
The Defaults That Reached Every Object Medium · Backend
The Deposit From Address Zero Medium · Backend
The Distribution List That Replied to Itself Medium · Backend
The Donation That Skipped the Health Check Medium · Backend
The Drain Order That Matched Too Much Medium · Backend
The Email Claim Anyone Could Set Medium · Backend
The Endpoint That Forgot Private Meant Private Medium · Backend
The Environment Variable That Ran Code Medium · Backend
The Exchange That Could Not List Itself Medium · Backend
The Feature File That Doubled Overnight Medium · Backend
The Fields You Never Saw Medium · Frontend
The Files Deleted Years Ago Came Back Medium · Database
The Filter That Held Back the Flood Medium · Backend
The Greedy Regex Medium · Backend
The Group Chat Nobody Approved Medium · Backend
The Internal Header Anyone Could Send Medium · Backend
The Key That Was Marked Keep Medium · Backend
The Label That Kubernetes Removed Medium · Backend
The Leaning Tower of Pins Medium · Frontend
The Leap Second That Went Negative Medium · Backend
The Log That Wore Out the Car Medium · Backend
The Login Key Made of Passwords Medium · Database
The N+1 Query Storm Medium · Database
The Node Update That Cut the Network Medium · Backend
The One-Cent Book of Madness Medium · Backend
The Open Redirect Medium · Backend
The Payment Method Called Nothing Medium · Backend
The Payment Page Script Nobody Pinned Medium · Frontend
The Phantom Unread Badge Medium · Frontend
The Phantom WHERE Clause Medium · Database
The Pointer That Ran Past the Buffer Medium · Backend
The Reducer That Bit the Hand Medium · Frontend
The Rotation That Ate the Backups Medium · Database
The Route Leak Nobody Filtered Medium · Backend
The Script That Granted Everyone Everything Medium · Database
The Term That Matched Every Route Medium · Backend
The Tip That Didn't Add Up Medium · Backend
The Toast That Never Left Medium · Frontend
The Token That Never Dies Medium · Backend
The Two-Terabyte Wall Medium · Database
The Update That Redirected Your Documents Away Medium · Backend
The Wrong Audience Medium · Backend
Three Checkboxes and $900 Million Medium · Frontend
Too Young by One Birthday Medium · Frontend
Touchdown at Forty Metres Medium · Backend
Tweets From a Borrowed Number Medium · Backend
Twenty-One Fields, Twenty Supplied Medium · Backend
Version 2.3.10 Is Not Older Than 2.3.9 Medium · Backend
When 65536 Read as Zero Medium · Frontend
Wildcards for Every Account Medium · Database
Zero Usage Meant Zero Quota Medium · Backend

Python (66)

Five Countries Missing From the Average Easy · Backend
Four Hours of Any Password Easy · Backend
Good Time on the Enhancement Easy · Backend
Nine A.M. Somewhere Easy · Backend
Nobody Left to Fly Christmas Easy · Backend
Pounds Where Kilograms Should Be Easy · Backend
The 1900 Leap Year Lie Easy · Backend
The Alarm That Slept In Easy · Backend
The Backup That Always Said Yes Easy · Database
The Deposits Nobody Reported Easy · Backend
The Draft That Overwrote the Publish Easy · Backend
The Ledger That Leaked Cents Easy · Database
The Metric Mars Mission Easy · Backend
The Mutable Default Trap Easy · Backend
The Negative Invoice Easy · Backend
The Notification Echo Easy · Backend
The Runaway Retry Easy · Database
The Uploader Nobody Checksummed Easy · Backend
Twelve Dollars for Google.com Easy · Backend
Two Digits Too Few Easy · Backend
Infinite Leverage Hard · Backend
The Backslash at the End of the Line Hard · Backend
The Block Only Half the Network Accepted Hard · Backend
The Fsync That Lied on Retry Hard · Database
The Lookalike Domain Hard · Backend
The Signature Nonce That Never Changed Hard · Backend
The Thruster That Spun It Faster Hard · Backend
The Track That Forgot Its Past Hard · Backend
The Username That Changed When Asked Twice Hard · Backend
alg: none Medium · Backend
Debts Averaged Out of Thin Air Medium · Backend
Default Country Roulette Medium · Backend
Every 256th Setup Medium · Backend
Half an Emoji in the Username Medium · Database
Locked Out of Everything Medium · Backend
Noon in Which Timezone? Medium · Backend
NULL Isn't Equal to NULL Medium · Database
Rounded Out of a Refund Medium · Backend
The Backup That Backed Up Nothing Medium · Database
The Cleanup Job That Ran Against Production Medium · Database
The Cursor at the End of Time Medium · Database
The Duplicate Check Skipped for Speed Medium · Backend
The Empty Argument List That Became Root Medium · Backend
The Fees That Priced People Out Medium · Backend
The Final Invitation That Never Came Medium · Backend
The Folder Called .GIT Medium · Backend
The Gene Named September 2nd Medium · Backend
The Generator That Vanished Medium · Backend
The Half-Applied Batch Medium · Database
The Hash That Started With a Zero Byte Medium · Backend
The Hashes Without Salt Medium · Database
The Index That Rounded Itself Poor Medium · Backend
The Installer That Deleted boot.ini Medium · Backend
The Invariant Off by a Hundred Medium · Backend
The Leap Day Meltdown Medium · Backend
The Loss That Lost Its Minus Sign Medium · Database
The Migration That Unblocked Everyone Medium · Database
The Missing Table Rows Medium · Backend
The Optimistic Lock That Wasn't Medium · Database
The Path That Climbed Out Medium · Backend
The Patriot's Drifting Clock Medium · Backend
The Replayed Webhook Medium · Backend
The Reset Mail Sent to a Look-Alike Medium · Backend
The Spreadsheet That Ran Out of Rows Medium · Database
The Streak That Died at Midnight Medium · Backend
Three Presses, Three Remittances Medium · Database

Java (55)

Gangnam Style Broke the Counter Easy · Backend
Read Your Own Writes Easy · Database
Sessions for Someone Else Easy · Backend
The 37th of March Easy · Backend
The Auction That Closed Instantly Easy · Backend
The Coupon That Stacked Itself Easy · Backend
The Dotless I in Istanbul Easy · Backend
The Request With Too Many Parameters Easy · Backend
The Share Price That Outgrew Its Integer Easy · Backend
The Sunset That Crashed the Phone Easy · Frontend
The Upsert That Forgot to Add Easy · Database
The Year 2038 Problem Easy · Backend
The Year That Didn't Fit in an Int Easy · Backend
Two Billion Events and One More Easy · Database
User Minus One Is Root Easy · Backend
Ariane's Sixteen Bits Hard · Backend
One Frame Behind Forever Hard · Frontend
Priority Inversion on Mars Hard · Backend
The Break That Left the Switch Hard · Backend
The Class Path Nobody Denied Hard · Backend
The Clock That Ran Backwards Hard · Database
The Hang at 49.7 Days Hard · Backend
The Log That Looked Things Up Hard · Backend
The Missing Torque Calibration Hard · Backend
The Root That Brought Its Own Generator Hard · Backend
The Signature Made of Zeros Hard · Backend
Three Runs Deep Is Not Deep Enough Hard · Backend
Two Billion Transactions Later Hard · Database
Two Waypoints, One Name Hard · Backend
248 Days to Reboot Medium · Backend
A Century of Missing Interest Medium · Backend
Escaping the Wildcard Medium · Database
Interest Sent as Orders Medium · Backend
Orders Sent to the Unknown Queue Medium · Backend
Random Routing, Idle Dynos Medium · Backend
Satoshis, Not Doubles Medium · Database
Tenths of a Second Since 2000 Medium · Backend
The App That Kept Registering Itself Medium · Backend
The Autosave That Cut You Off Medium · Frontend
The Blank Field That Crashed Every Region Medium · Backend
The Certificate Nobody Renewed Medium · Backend
The Clock Started Eleven Hours Early Medium · Backend
The Consumer Key That Opened Enterprise Mail Medium · Backend
The Edge of the Far Lands Medium · Backend
The February 29th Certificate Medium · Backend
The Kill Screen at Level 256 Medium · Frontend
The Leaderboard Shuffle Medium · Frontend
The Midpoint That Overflowed Medium · Backend
The Negative Shard Medium · Database
The Percent That Escaped Medium · Backend
The Storage Certificate That Lapsed Medium · Backend
The Week Number That Went Back to 1999 Medium · Backend
The Week That Belonged to Next Year Medium · Backend
The Zune Freeze Medium · Backend
Two Files Named classes.dex Medium · Backend

How it works

Read the report. Every hunt opens with the bug report and the project files. No line numbers, no hint about where — finding it is the exercise.
Trace and fix. Edit the files in the workspace. The tests are hidden; run them as often as you like to see how many pass.
Submit the fix. A complete fix awards 50 XP, counts as an active day for your streak and appears in your submission history with the code you sent.

Who it is for

Developers who want the skill interviews and jobs actually test — reading and reasoning about someone else's code — rather than another blank-function exercise.

Frequently asked questions

How is a bug hunt different from a challenge?: A challenge gives you a signature and asks for an implementation. A hunt gives you an implementation that already exists and asks what's wrong with it. The skill it trains is reading and reasoning about someone else's code.
Are the tests visible?: No. You see the bug report and the code. Hidden tests decide whether your fix is complete — partial fixes show as a partial pass count.
Can I filter hunts?: By difficulty and by category, so you can deliberately practise the kind of bug you're worst at.
Do hunts affect my rating?: Yes. The first complete fix of a hunt pays 50 XP and adds 50 to your rating, and it keeps your streak alive for the day. Fixing it again is practice and pays nothing more.