How to Reproduce a Bug

A ticket lands: "App crashes sometimes." Or a teammate leans over: "It's broken, can you look?" You open the code, you stare, and nothing is obviously wrong. The dread that follows isn't really about the bug - it's the sinking feeling that you have no way to get at it. You can't see it happen, so you can't tell if it's fixed, so you're poking in the dark and hoping.

Here's the relief: almost every "I can't fix this" is actually "I can't reproduce this yet." Reproduction is the skill that turns a ghost story into a controllable experiment - something you can trigger whenever you want, watch closely, shrink down, and finally confirm dead. Get good at this one thing and the rest of debugging stops being scary. This guide shows you how.

How to read this

Fighting a bug that won't show itself right now? Jump to Phase 3: When It Won't Reproduce (Heisenbugs) - the cheat-card at the top maps each "it's intermittent" symptom to a tactic.
Want the skill to finally click? Read in order. Each phase builds the one before it: why reproduction is the whole game, how to pin a bug down, then what to do when it fights back.

The phases

Why Reproduction Is the Whole Game - you can't fix (or prove you fixed) what you can't trigger on demand. The mental model: make it happen reliably, then shrink it.
Nailing It Down - the four variables that actually matter (steps, environment, data, state/timing), and how to build a minimal reproduction by removing everything that isn't load-bearing. Includes "works on my machine," decoded.
When It Won't Reproduce (Heisenbugs) - the intermittent ones: the usual culprits (timing, uninitialized state, external dependencies, caching) and the tactics that drag them into the light.

Deliberately deferred to follow-up guides: how to fix a bug once you can trigger it (see Using a Debugger), reading the crash output it produces (see Reading a Stack Trace), and hunting down which commit introduced it (see Bisecting a Bug). This guide is only about getting the bug to happen on command.