HAha…NEVER admit your fault to this question! It’s just a mind game. This is a good time to show your humorous side, show them that you have a personality…catch them off guard and change the topic.
The worst is listening to someone ramble on about a time they fucked up. It’s like being forced to read a drunken Sleeper emo post that makes no sense. If you feel that you have to cite a situation, recall an actual experience related to your career path. Don’t make anything up, people can tell you are lying through your teeth. Body language is a big thing, show you have confidence in yourself.