Practice catalogue updated Wed, Sep 09, 2026
Debug the signal.
Ship the fix.
Production-shaped debugging labs across APIs, config, concurrency, and infrastructure. Reproduce the failure, follow the evidence, and prove the smallest safe change.
Ready labs145playable now
Quick starts100under 20 min
Avg. time19minper lab
OSS labs4public history
01Reproducemake the failure visible
02Tracefollow the evidence
03Patchchange the smallest thing
04Verifyprove it holds
Featured labBuglyst · Events & Concurrency
Await Missing in Fire-and-Forget
Batch runner swallows async failures because the per-item call is not awaited.
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Focused bugs are targeted exercises; codebase incidents are deeper, production-shaped investigations with architecture, logs, and multiple layers. Search by symptom, then filter by lab type, source, status, or difficulty.
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116Focused bugAwait Missing in Fire-and-ForgetFeaturedBatch runner swallows async failures because the per-item call is not awaited.3 attempts · 2 solved · 0 discussionsEasy12min01Focused bugThe Phantom Env VarWorks locally. Fails in CI after a config change.50 attempts · 17 solved · 1 discussionsEasy12min02Focused bugWrong Port, Right ServiceHealth check passes, traffic hits the admin service instead of the public API.28 attempts · 17 solved · 1 discussionsEasy14min03Focused bugThe Stale LockDeploy pipeline stuck because an old lock never expired.18 attempts · 9 solved · 1 discussionsEasy15min04Focused bugAccessDenied at 3AMProduction job fails only after rotating credentials.17 attempts · 4 solved · 0 discussionsMed.22min05Focused bugThe Silent TimeoutRequests hang for 30 seconds, then succeed on retry.8 attempts · 0 solved · 1 discussionsMed.24min06Focused bugCache StampedeEvery morning at 9AM service slows to a crawl for 90 seconds.4 attempts · 0 solved · 1 discussionsMed.28min07Focused bugThe N+1 That Wasn'tDB load explodes, but the query count looks normal.4 attempts · 1 solved · 1 discussionsMed.26min08Focused bugThe Missing HeaderAPI works in browser, fails in worker/client.22 attempts · 9 solved · 1 discussionsEasy13min09Focused bugThe Flaky RetryRetry logic makes the outage worse.4 attempts · 2 solved · 1 discussionsMed.20min10Focused bugThe Broken Health CheckDeployment rolls back even though app is alive.7 attempts · 3 solved · 1 discussionsEasy16min11Focused bugThe Timezone TrapReport fails only around midnight UTC.4 attempts · 0 solved · 1 discussionsMed.23min12Focused bugThe Case-Sensitive ConfigWorks on macOS, fails in Linux CI.10 attempts · 2 solved · 0 discussionsEasy10min13Focused bugPagination off by onePage 1 skips the first N items.97 attempts · 29 solved · 0 discussionsEasy10min14Focused bugCORS origin parsing bugRegex origin validation allows malicious domains.5 attempts · 2 solved · 0 discussionsMed.15min15Focused bugCache key missing tenant idMulti-tenant cache key uses just user_id, causing cross-tenant leaks.4 attempts · 1 solved · 0 discussionsHard20min16Focused bugThe stale confirmationA successful update is followed by an old value.2 attempts · 1 solved · 0 discussionsMed.18min17Focused bugThe missing trace IDA downstream request loses its correlation header.3 attempts · 3 solved · 0 discussionsEasy14min18Focused bugHalf an importA rejected batch still leaves earlier rows committed.2 attempts · 0 solved · 0 discussionsHard24min19Focused bugThe log that ate the requestA debug log serializes an entire upload payload.1 attempts · 0 solved · 0 discussionsMed.20min20Focused bugThe Price That Refused to UpdateMerchants change a price. Half the storefront still shows yesterday's number.8 attempts · 3 solved · 0 discussionsEasy12min21Focused bugThe Permission That Outlived the UserSecurity revokes a contractor. API logs still show successful deletes.1 attempts · 0 solved · 0 discussionsMed.15min22Focused bugThe Invoice That Forgot QuantityCarts with quantity > 1 undercharge. Finance finds missing line totals.1 attempts · 0 solved · 0 discussionsMed.15min23Focused bugThe File That Left the VaultA document download endpoint serves more than the vault folder.5 attempts · 3 solved · 0 discussionsMed.18min24Focused bugThe Dashboard That Averaged AveragesOn-call swears p95 is fine. Customers still time out.5 attempts · 4 solved · 0 discussionsMed.15min
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