The biggest interview mistakes tend to be:
(1) not having a good answer for "why medicine", or simply falling back on the cliche of wanting to "help people". Similarly, doing it for money, prestige, parent influences or other superficial reasons is a no-no
(2) not having a good answer for "why this school"; Telling them you want that school because it is in a particular city is not a good answer.
(3) Coming off as not well thought out.
(4) Coming off as immature
(5) Not being responsive, and having the interviewer feel like s/he is pulling teeth to get you to talk.
(6) Being to arrogant, or giving off an air of entitlement,
(7) Not having any questions to ask -- it suggests disinterest.
(8) Not being nice and professional toward non-interviewer admissions staff, secretaries, student tour guides, student hosts, etc.
(9) Not looking professional -- this is not the time to take fashion risks.
These are the ones that come to mind, I'm sure there are others.