The interview was going so well..
You were clicking on all cylinders. They were listening intently to all your answers and seemed to love them.
That one thing came out wrong. You could hear the words come out of your mouth and shortly after your brain was trying to send an urgent message to the mouth. "Stop! Don't go down that rabbit hole!"
But, it was too late. The genie was out of the bottle. Then, the interviewer started asking follow up questions to probe deeper. The whole interview was coming unraveled.
Many of us have the not so fond memories of blowing an otherwise great opportunity. This happens far more frequently than anyone realizes. Most of the time the candidate never even realizes the grave error he made. After the interview he likely thought it went really well.
The main reason this happens is lack of preparation. The candidate hasn't anticipated the questions that will go the wrong direction. He hasn't learned techniques to proactively steer the interview to highlight strengths. He says the things he always says without ever realizing how they are received.