TL;DR: Pick something with real consequence that you caused, own it without theatre, and spend most of the answer on what you did in the minutes after you realised and what changed structurally. The scoring is on disclosure speed and on whether the fix was systemic rather than a promise to be more careful.
How to approach it
Choose deliberately before you speak. The mistake must be yours, have had a visible consequence, and be far enough back that the fix is verifiable. Then follow the order below, because the sequence carries most of the signal.
A strong answer
Name it plainly. A hypothetical example, to replace with your own experience: "I ran a migration against production believing it was staging, and locked the orders table for eleven minutes during business hours." No preamble, no shifting into the passive voice. Interviewers notice the grammar: "the migration was run" is the sound of someone who has not accepted it.
Say when you realised and what you did in the next sixty seconds. This is the highest-value part. Did you stop, or did you hope? Did you tell someone immediately, or did you try to fix it before telling anyone? Disclosing fast is the behaviour being tested, because a mistake that the team learns about early is recoverable and one they discover an hour later usually is not.
Describe the mitigation honestly, including anything you got wrong under pressure. If you made it briefly worse before making it better, say so. That detail is rarely invented and it reads as true.
Then the structural change, and this is where most answers fail. "I was more careful afterwards" is not an answer, because it does not survive you leaving. Choose controls matched to the failure: restrict production credentials, require an authorized migration pipeline and verify the target environment before execution. Confirmation prompts and terminal colours can reduce confusion but do not eliminate wrong-environment mistakes. State the remaining risk.
Close with the second-order effect if there was one. Did you write it up? Did anyone else's practice change? A blameless postmortem that produced changes across a team is a stronger ending than a personal lesson.
Choose an example you can discuss truthfully within confidentiality constraints. Do not invent a smaller event or conceal material facts to make the story safer. Explain the consequence, your response and what changed; anonymize sensitive details when necessary.
And do not blame the system in a way that removes your agency. "There were no guardrails" is true and it is the fix, not the excuse. The order matters: I did this, here is what happened, and here is the guardrail that now exists so nobody can repeat it.
What interviewers probe next
"Why did you have production access from a laptop?" Answer factually. This is where a systemic finding is legitimate, and it demonstrates you understood the cause rather than just the event.
"How did your manager react?" They are checking psychological safety and whether you would disclose again. If the reaction was bad, say what you learned about where you would work.
"What is the smallest version of this that still happens?" A good question and a good answer names the residual risk honestly rather than claiming it is solved.
Common mistakes
The humble brag, where the mistake is caring too much or shipping too fast. Interviewers score it as an inability to self-assess.
Ending at "I learned to double check", which is a promise rather than a control.
Diffusing responsibility across the team. The question said "you", and the answer that keeps saying "we" is heard clearly.