Tell me about a time you received a complaint and how you handled it

In my first year, a QA engineer told me that a feature I had built was frustrating to test because I had made several assumptions that weren't documented and one edge case caused the screen to fail silently. I thanked them for flagging it and asked them to walk me through exactly where they got stuck, because at first I thought it was just one bug. That conversation showed me the bigger issue was that I had optimized for getting the feature working, but not for making the behavior clear to someone else. I fixed the bug, added clearer error handling, and updated the handoff notes with the assumptions and test cases I had missed. I also checked back with them the next day to confirm the updated flow was easier to test. After that, I started including a short list of edge cases whenever I handed off work, and reviews went much more smoothly.

At my last job, I was helping with support for a small e-commerce team, and a store owner called in upset because a discount code they had emailed customers was not working during a weekend promotion. I listened without interrupting, apologized for the inconvenience, and asked a few questions so I could understand exactly what they had tried and how many orders might have been affected. I found that the code had been entered correctly, but the expiration date had been set in the wrong time zone, so it had shut off a few hours early. I fixed it right away, helped them test a new code before sending anything else, and then wrote a simple checklist for our team so we would confirm time zones before scheduling future promotions. I also followed up later that day to let them know what had happened and what we had changed. After that, I became a lot more careful about how I handled customer-facing settings, because I saw how much trust even a small mistake can affect.