I think when you're just starting that first race condition or memory management issue are exciting. After a while it's not that exciting any more. I got asked that question and the first thing that popped to my mind was a very unexciting bug I was chasing in legacy code I was maintaining. I don't think the interviewer liked that answer because it was mostly grunt work and not some some amazing feat of engineering.

I don't think it's a particularly good open ended question for more experienced developers and an interview is not a camp fire circle... It could be part of an investigation in some past project, e.g. what problems did you have and how did you solve them...