I don't think that's necessarily the case, we're just having a hard time understanding your goal. If all you want to do is see what future classes would be like, most admissions directors would gladly set you up with a student that you could hang out with for a day. If you're looking for more of an educational edge, then you'd definitely be better off reviewing stuff on your own (you likely wouldn't get enough out of lecture to make it worth your time).
Personally, I wouldn't care a bit if you came and sat in on my classes for whatever reason (UNLESS you started making lecture last longer by asking questions, that is). Would instructors find out? Unlikely since all of my classes have been "team-taught." But, students would definitely notice, and God only knows what some med students are capable of when they think someone's trying to get an advantage over them (not that this is what you'd be doing).
Anyway, take this for what it is--random people giving their opinions to your question. Take all responses with a grain of salt, sort through them, and try to find the advice you were looking for. Good luck.