I was accepted at my top choice after having an "average" interview. At least that's how I felt about it. They were three interviews back-to-back. My first interviewer was unfriendly, had a stern face, and spent the majority of the interview trying to highlight all the deficits in my app. At one point I got so nervous I froze up and blurted out "Sorry, I'm just REALLY nervous right now." Womp wommmp. The second interview went okay, but when she asked me if I had any more questions, I froze and couldn't remember the list of questions I had prepared to ask. The interview was also very short--only 20 minutes instead of the allotted 30 minutes. In my third interview, the interviewer asked me specific questions about some research I did a few years ago and I fumbled and didn't answer the questions well. He also asked me this weird theoretical question and I gave him the complete wrong answer to it (I checked after the interview.)
That being said, I think that when we go over the interviews in our head we focus on our hiccups, and not on all the things that we answered well. A lot of times they're looking to see if you are personable, articulate, and a good fit for the school. If you are all three of these things but have a few slip-ups that may not make or break you depending on the school and what they're looking for.