Tell me about a project that did not go as planned — a genuine failure or a serious mistake. What happened, and what changed afterward?
Practice against the follow-up probes
- What was YOUR specific contribution to the failure — not the team's, not circumstance's?
- When did you first suspect it was going wrong, and what did you do with that suspicion?
- How did you communicate the failure upward and to affected teams?
- What concretely changed — in your process, not just your awareness — because of it?
- Would the same failure be caught earlier today? By what mechanism?
Show answer guide
What the interviewer is probing
Whether you can be trusted with ambiguity and bad news. The failure question is really four questions: do you take real ownership (the sanitized "we underestimated complexity" is a dodge), do you detect failure early or ride it down (the "first suspicion" probe), do you communicate it before you're forced to, and did it produce a mechanism rather than a resolution to be more careful. Interviewers also calibrate the failure's size: a trivial stumble suggests you've either never owned anything consequential or won't share the real stories — both disqualifying at senior levels.
How to structure your answer
- Pick a failure with real stakes and real personal responsibility — a project YOU drove that didn't deliver, not a team misfortune you witnessed.
- Tell the setup straight: the bet, why it was reasonable at the time, and the early signal you missed or rationalized — naming the rationalization is where credibility is earned.
- Narrate the turn: the moment you knew, the gap between knowing and acting (be honest about it), and how you communicated — especially the hard upward conversation.
- Describe the recovery: what you salvaged, how you unwound commitments, what you did for the people affected.
- Close with the mechanism: the specific practice that exists now because of this — a kill-criteria ritual, a pre-mortem habit, a milestone that forces the go/no-go earlier — and ideally one later instance where it fired.
Strong signals vs. weak signals
- Strong: consequential failure; first-person ownership of a specific error in judgment; honesty about the delay between suspicion and action; proactive bad-news delivery; a named mechanism with evidence it later worked.
- Weak: a humble-brag ("we only 2x'd instead of 5x'd"); ownership diffused across the team or blamed on shifting requirements; failure discovered by others; the lesson is "communicate more" or "I learned so much" with no mechanism; or visible discomfort that suggests the real stories are being withheld.
Source
Curated from real senior data science and ML interview loops.