Describe a time when your production build caused a major UI issue — what was your response and fix
Behavioral STAR-style: name a real incident (e.g., chunk hash mismatch after deploy → blank page; service-worker caching stale bundle → users stuck on broken version; CSS purge dropping a runtime-only class). Cover detection (monitoring alert / user reports), immediate mitigation (rollback / kill-switch), root cause, fix, and what changed in the process so it won't recur.
For a behavioral question, walk it through STAR — Situation, Task, Action, Result — and end with what you changed in your process. Below is a pattern; substitute a real incident.
A template you can adapt
Situation. "We shipped a release on a Friday afternoon (lesson #1 right there). Within 20 minutes, support tickets started — users were seeing a blank page after navigating between routes."
Task. "I was on-call. Goal: stop the bleeding for current users, find the root cause, and ship a fix without making it worse."
Action.
- Detect — Sentry showed a spike in
ChunkLoadError. The release shipped new code-split chunks; users on the previous page had old chunk references in memory, but the new deploy invalidated those URLs (CDN immutable hashed assets). Their next route transition failed because the chunk URL 404'd. - Mitigate immediately — added a catch-all error boundary that detected
ChunkLoadErrorand prompted "We've updated this page. Reload to continue." This was deployed within 30 minutes and stopped the blank-page reports. - Root cause — our CDN configuration was deleting old hashed assets on deploy. The intent was cleanup, but the effect was that any tab open during a deploy lost access to its current chunks.
- Permanent fix — kept old hashed assets around for 7 days post-deploy; also added a
controllerchange-driven "update available" prompt so users moved to the new version cleanly. - Communication — posted incident updates in a customer-facing status page and an internal Slack channel.
Result. Spike resolved within 30 minutes. Zero permanent data loss. Post-mortem ran the next week.
What changed.
- Deploy retention policy: old assets kept 7 days.
- "Reload to update" UI on chunk errors and
controllerchange. - Sentry alert specifically on
ChunkLoadErrorrate, paging on threshold. - No deploys after 3pm Friday or before 11am Monday.
Why this pattern works
- Detection shows you instrument production.
- Mitigation first shows you stop the bleeding before chasing the cause.
- Root cause demonstrates technical depth.
- Process change shows you don't fix one incident — you fix the class of incident.
Other incident archetypes you can use
- Service-worker shipped a broken bundle, kept serving it because the SW didn't update — fix: kill-switch, skipWaiting + reload prompt.
- CSS purge dropped a class used by a JS-runtime-rendered modal — caught only after deploy because the test suite didn't render that modal — fix: safelist plus visual-regression test for the missing path.
- Environment variable misconfigured — feature flag defaulted to "off" instead of "on" in prod after a refactor — fix: typed config schema with required-at-build, plus a smoke test that hits the flag.
- Cache invalidation — backend ETag bug served stale data to all users — fix: cache-buster on the rollout, and a "version dropdown" in support tooling.
- CDN edge mis-routing during a config change.
What interviewers are listening for
- You take ownership ("I").
- You stop the bleeding before debugging.
- You can articulate the root cause precisely.
- You fix the class of bug — not just this one occurrence.
- You communicate with users and the team during the incident.
- You don't blame ("the deploy was bad", not "Bob's PR was bad").
- You learned something — and the org learned something too.
Pitfalls to avoid
- A vague "everything broke and we fixed it" — name the technology and the exact failure mode.
- Skipping the mitigation step — recruiters notice when candidates jump straight to "we fixed it."
- Making it sound easy — incidents are stressful; show that you were thoughtful under pressure.
- Forgetting the lesson — a great story without a process change isn't great.
Interview framing
"I'd ground it in a specific incident — for me, the canonical one is a chunk-load failure after a deploy: old tabs held references to chunks the CDN had already deleted, so navigation broke with a blank page. I'd walk through detection (Sentry spike on ChunkLoadError), immediate mitigation (an error boundary that prompted reload — stopped the bleeding in 30 min), root cause (CDN deleting old assets on deploy), permanent fix (7-day retention + controllerchange update prompt + a Sentry alert on chunk-error rate), and the process change (no Friday-afternoon deploys, paged alerts on this specific error). The pattern matters more than the specific incident — mitigation first, root cause, fix the class of bug."
Follow-up questions
- •What signal told you something was wrong?
- •How did you decide rollback vs forward-fix?
- •What did you communicate to users during the incident?
- •What process changed afterwards?
Common mistakes
- •Blaming a person or 'a bad PR'.
- •Vague description without technical specifics.
- •Skipping the mitigation step.
- •No follow-up process change — looks like nothing was learned.
- •Making yourself the sole hero — real incidents are team responses.
Performance considerations
- •N/A — behavioral.
Edge cases
- •Incident where the cause was a third-party (CDN, vendor).
- •Incident where rollback wasn't possible (DB migration).
- •Incident that turned out not to be your team's fault — show how you helped find that.
Real-world examples
- •Chunk-load failure after deploy.
- •Service-worker stale bundle.
- •Tailwind/CSS purge dropping classes.
- •Feature flag default flipped during refactor.
- •CDN cache poisoning.