If it does happen, don't assume that it was due to poor performance in the interview.
Some schools use a formula for admissions and interviews count as just one part of the formula. So if you fail to get in, it may be due to aspects other than the interview as well.
So the weighting in terms of importance might be: 1/3 PCAT, 1/3GPA, 1/3 interview. Perhaps you had a decent interview but the overall score was not strong.
Other schools grant interviews after a certain baseline measure is achieved on other aspects of the application. From that point on, admission is based on mostly the interview.
The point is, if you do get an outright rejection after interviewing somewhere, don't assume it was because of the interview itself, although it should go without saying that improving on interview skills may prove beneficial.