We'll I'm sure we've all had a bad experience at an interview, either being interviewed or being the interviewer.
I'll tell you one of mine:
I was interviewing a guy for a role of Senior Business Analyst, quite a few years ago now (to save the embarrassment, let us call him John).
So, John came in sat down and the interview kicked off quite well, asking the usual questions about his skills and experiences to date. All of a sudden John very distracted, so i asked him is everything ok? I got a quite short answer back 'Yup!' and I could see he was in a little distress.
I was about to ask if he needed a water or something when he almost shouted "SORRY" and shot up out of his chair and ran out of the room, down the stairs and out of the office. My colleague and I were shocked thinking was it something we did or said to have such a reaction.
After the other interviews, we had lined up I called the agency of John and told him what happened and what was wrong.
John, had apparently had a little but of some dodgy food and had a little accident in his trousers, and was too embarrassed to say anything and just ran so we didn't notice anything.
Poor guy, I felt really sorry about how it ended and asked for another interview with him as he had all the credentials we were looking for, however, he was too embarrassed to come back.