Practice catalogue updated Fri, Sep 25, 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 · CI/CD & Pipelines

The Failure That Disappeared In The Aggregate

A failed continuable step is recorded but the stage aggregate still reports ok:true.

Hard26 minJavaScript#track#ci-cd-pipelines#continue-on-error

all labs · ready to play

Choose a failure mode.

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.

press / to search

15 of 145 labs · "Database"ready labs · catalogue order

#LabDifficultyTime

07Focused bugThe N+1 That Wasn'tDB load explodes, but the query count looks normal.6 attempts · 2 solved · 1 discussionsMed.26min 16Focused bugThe stale confirmationA successful update is followed by an old value.4 attempts · 1 solved · 0 discussionsMed.18min 27Focused bugThe Query That Loaded the WorldOrders list endpoint returns every tenant's rows and times out under load.0 attempts · 0 solved · 0 discussionsMed.15min 29Focused bugThe Config That Swallowed ProdProduction boots with the dev database URL and debug flags.9 attempts · 3 solved · 0 discussionsMed.15min 62Focused bugCache invalidation skipped on writeWrites update the database but the cache is never told, so reads keep returning the old value.0 attempts · 0 solved · 0 discussionsHard28min 68Focused bugThe Wallet That Let It SlideTwo concurrent transfers from the same account both succeed and the balance goes negative.2 attempts · 0 solved · 0 discussionsEasy14min 69Focused bugTwo For One EmailConcurrent registrations for the same email both succeed and create duplicate accounts.0 attempts · 0 solved · 0 discussionsMed.18min 72Focused bugThe Phantom ReadA read inside a transaction does not observe the same transaction's pending write.0 attempts · 0 solved · 0 discussionsMed.18min 106Focused bugInner join drops comments from soft-deleted authorsA LEFT JOIN needs an INNER JOIN — or vice versa — and rows silently disappear from the listing.1 attempts · 0 solved · 0 discussionsEasy15min 107Focused bugCount that missed the where clauseA dashboard count helper ignores its filter and reports the wrong total.4 attempts · 1 solved · 0 discussionsEasy12min 108Focused bugN+1 that silently drops rowsA per-row fetch helper filters on a property that does not exist, so every caller sees an empty result.2 attempts · 1 solved · 0 discussionsMed.20min 109Focused bugPagination skips rows on timestamp tiesCursor-based pagination drops every row whose updated_at equals the boundary timestamp on the previous page.5 attempts · 0 solved · 0 discussionsMed.22min 110Focused bugPseudo-unique constraint leaks duplicate accountsAn index assumed unique is queried by the wrong column, so duplicates slip through signup.0 attempts · 0 solved · 0 discussionsHard28min 134Codebase incidentThe pool that never gives connections backA read API works locally, then queues and returns 5xx under sustained traffic as the database pool drains.3 attempts · 0 solved · 0 discussionsHard35min 143Focused bugIdempotency key ignoredThe payment endpoint validates an idempotency key but drops it before the database call.3 attempts · 1 solved · 0 discussionsMed.20min

discussion · public Practice

What are developers debugging?

Questions, dead ends, and useful investigation clues from real public Practice threads.

latest threads

Loading discussion threads…

real activity · public practice

What developers are doing.

Recent public activity from developers who chose to share their Practice work.

Loading recent public activity…

keep building the loop

More ways to practice.

Move from a focused drill to open-source history or company-style patterns when you want a different kind of debugging pressure.

Focused exercises

Train code review, system tracing, CI diagnosis, incident response, and fast debugging instincts.

05 drills5 formats · short sessions

Open-source labs

Practice against safely recreated public bug-fix history from the libraries that actually broke.

04 playablereal bug-fix history

Company-style prep

Train backend and infrastructure failure patterns without affiliation claims.

30 patternsPattern-based · unofficial