Why interviewers ask this question
Interviewers ask this to see whether you take genuine ownership of errors, learn from them, and communicate honestly about failure rather than deflecting blame elsewhere. Everyone makes mistakes at work; the answer that concerns interviewers is not the mistake itself but a candidate who cannot identify a real one, blames someone else for it, or shows no evidence of having changed their approach afterwards.
How to structure your answer using STAR
Pick a genuine mistake with real consequences, but not one so severe it raises doubts about your judgement for the specific role you are interviewing for. Briefly explain the situation and what went wrong, then spend most of your answer on the action: how you identified the mistake (ideally before someone else had to point it out), how you communicated it honestly to the people affected, and what you did to fix it or limit the damage. Finish with what you changed afterwards, showing a concrete process or habit you now use specifically because of that mistake.
A strong example answer
"Early in a previous role, I sent a client report using last month's figures instead of the updated numbers, because I had copied a previous template without double-checking every field. I noticed the error myself a few hours later, contacted the client immediately to explain and correct it before they had acted on the incorrect figures, and apologised directly rather than waiting to see if it would be noticed. Since then, I built a simple checklist I run through before sending any client-facing document, and I have not repeated that particular mistake since."
Mistakes to avoid
Avoid the common trap of describing a "fake weakness" mistake that is really a strength in disguise, such as "I work too hard and missed a deadline because I took on too much," since interviewers see through this immediately and it can come across as evasive. Avoid blaming a colleague, unclear instructions, or bad luck as the main reason, even if partly true, since the question is specifically testing ownership. Never end the answer without a clear, specific change you made afterwards.