Bombed more than one technical interview but here's one example.

Phone screen interview with Google. Interviewer asked about how I would keep track of the nth highest value in an infinite stream of values. I said I would use a heap. Then interviewer asked to me write the code for that heap in a Google Doc. I can't remember what I wrote, but I'm pretty sure that I barely got started. Boom.

Lesson here is that simply knowing something in general might not be enough. If you think you know something, try implementing it from scratch to be sure.