I went through this whole process last year and can say that there were certainly interviews where this happened. I was pleasantly surprised after one where I was asked an ethics question that I totally bumbled and said, "This is the dilemma, but no, I don't remember the specific ethic" and shrugged. My interviewers later emailed me after the match results and said I was one of the best applicants they had had and they hoped I would reapply if I did not match. Not much of this whole process is actually predictable. I had sites tell me that I seemed like a great match for them, but then when I ranked them high up on my list, I didn't end up matching at all. It's not worth worrying about because so little of this is actually in our control. You can have the most perfect cover letter, 600+ intervention hours, have great recommendation letters, get a great feel for the site, and still not match. Or you can get zero sense from the interviewers whether they liked you or not only to later learn that you left a great impression. Don't beat yourself up.