What do you do with a bug you saw once but can't reproduce?
BugsMid-level
In shortWrite down everything I remember right away, save any evidence such as logs and recordings, then try to reproduce it by varying one condition at a time. If it still won't reproduce, I report it honestly as intermittent with the frequency and evidence, rather than dropping it.
Model answer
Bugs that happen “sometimes” are often the most serious ones in production, so I don’t ignore them.
1. Capture what I have, immediately
- Notes: exact time, build, device, account, what I did just before, network.
- Logs before they’re overwritten: browser console and network tab,
adb logcaton Android, device logs on iOS. - A screenshot or, ideally, a screen recording. I often keep recording running during exploratory sessions for exactly this reason.
- Check the crash reporting tool or server logs for that time.
2. Think about what could make it intermittent
- Timing: double taps, slow responses, a race between two requests.
- State: first launch vs later, logged in for a long time, app returning from background.
- Data: a specific account, item, long text, special characters, an expired token.
- Environment: network switch, low memory, another device or OS version, a time zone or date.
3. Vary one condition at a time
Repeat the steps with a slow network, then with a different account, then after backgrounding the app. Changing one thing at a time tells me what matters.
4. Report it honestly
If I still can’t reproduce it reliably, I report it as intermittent: “Seen 1 time in about 20 attempts”, with all evidence and what I already tried. I ask the developer to look at the logs; sometimes the stack trace alone is enough to find the cause.
Example: a cart sometimes showed an empty list. After trying different things, it happened only when I tapped the Cart tab within a second of adding an item. It was a race between two API calls, which the developer fixed once I gave him that detail.
Learn more: How to Write a Bug Report
What they're looking for
A systematic approach to intermittent bugs, knowledge of typical causes (timing, data, state, network, device), evidence gathering, and honest reporting instead of hiding or exaggerating the bug.
Common mistakes
- Ignoring it because it only happened once.
- Reporting it as "always" reproducible to get attention.
- Changing many conditions at once, so you never learn what matters.
- Not saving logs right away, before they're overwritten.