Yes, this is a good question to ask.
If the person you are asking is really foundfond of testing he would be very exitedexcited to tell you the story. It does not matter whether he is a Manual Tester or an Automation Engineer, the oneboth should have something to be proud of (but we should consider that he is telling the story from his point of view, I mean this bug could be not that impressive for a Automation Engineer but a kind of great bug among Manual Testers team)
And here is my example: memory leak defect.
When I had just been employed there was a problem in software: sometimes it crashed in an unpredictable manner (sometimes during one action, sometimes during another action). At that moment I was testing mainly with QTP (now UFT) and started to try Java-tests. The advantage was great: I could check JVM-memory during execution and dump memory usage right at the crash time (with MemoryAnalyzer-1.6.0). After analyzing memory it was clear that there were 8 objectobjects that were duplicated many times during program execution. So I reported defect with certain object names. Devs liked that because it was supposed that there is nothing to do with this unpredictable crashes and localization of the bug was of a top level.