X broke? Read Y

Goal

Go straight to the right module when something goes wrong in production, without reading the whole guide.

Use this page as a reverse index. Find the symptom, read the likely cause and go to the module. Modules that aren’t written yet appear without a link.

Slow

Symptom	Likely cause	Where to read
Fast locally, slow in production	Network latency multiplied by many database round trips (N+1)	1.3, M11
A route gets slower as the table grows	Missing index, large OFFSET, SELECT *	M10, M11
Every request stalls at the same moment	Event loop blocked by CPU work or synchronous I/O	M06
Slow at peak times, fine the rest of the day	Exhausted connection pool, no caching	M25, M26
Great p50, terrible p99	Contention, GC, a slow external dependency	M26, M17
Page loads slowly even though the API is fast	Large bundle, wrong rendering strategy, images	M08

Wrong data

Symptom	Likely cause	Where to read
Duplicate records (charge, email, order)	A non-idempotent operation being repeated	2.2, 2.3
A balance or counter is “sometimes” wrong	Race condition (read, compute, write)	1.3, M10
User sees old data right after saving	Cache without invalidation, lagging replica	M25, M27
Data disappears after a deploy	State kept in process memory	2.1
A migration locked the table in production	ALTER TABLE taking a heavy lock	M12
Webhook processed twice or out of order	At-least-once delivery treated as exactly-once	M24, M28

Down

Symptom	Likely cause	Where to read
An external service goes down and takes your app with it	No timeout, unbounded retries, no circuit breaker	M17
Worked in staging, broke in production	Environment differences (config, data, scale)	1.2
A deploy broke and you can’t go back	No rollback strategy, irreversible migration	M22, M12
The domain stopped resolving after a change	DNS and TTL	M14
A queue keeps growing	Slow consumers, no backpressure	M24
The build broke “on its own”	Dependency without a pinned version	M09

Secure?

Symptom	Likely cause	Where to read
A user sees someone else’s data by changing the ID in the URL	Missing per-resource authorization (IDOR)	M16
An API key showed up on GitHub or in the bundle	Secret in the wrong place	M16
An npm package behaves strangely	Supply chain attack	M09
A user manipulates your app’s LLM	Prompt injection	M33

No idea what’s going on

Symptom	Likely cause	Where to read
A production error that won’t reproduce locally	No structured logs or tracing	M19, M34
You find out it’s down when customers complain	No alerts or SLOs	M19, M35
A bug showed up and nobody knows since when	History not put to use	M04 (git bisect)
A simple change breaks something far away	Coupling, wrong abstraction, missing tests	3.3, M18, M23