possible acceptance retract ?

If they made you an offer, they will not retract it.

EXCEPT, there will come a time around June or July when they get lists from AMCAS of accepted students, and acceptees holding other places as well. Since schools like to firm up the class, and there are others on the wait list, you may get a letter giving you a deadline, commonly two weeks, to make up your mind whether you will come to them or not. If you don't respond with a yes, you will be dropped. If you say yes, other med schools you are holding will be informed that you have accepted another offer.
What you should be doing all along is each time you get a second offer, make up your mind, then keep the one you prefer, withdraw from the other. When you were on interview, you should have spent at least as much time to find out all you could about the medical school and whether you would want them, as they spent on finding about you and whether they will want you. Otherwise, when the crunch comes and you have to choose between two or more offers, how will you decide? Ask about rankings? Ask someone else's opinion? Return to take a second look? How?

And except in the rare event an error was made and you received an acceptance meant for someone else. It will be their decision as to whether or not they will retract the acceptance.

 

I think I should point something out here in response to gower's comment: To my understanding, even if you are offered an acceptance in error, the school must honor it. I think it is against the law to do otherwise. If it isn't, then there is at least the possibility of legal action. I know because it happened just last year at the school I'm going to attend.