As far as I know, one of the biggest and most oft-quoted misquotes would be this one from Hamlet:
'Alas, Poor Yorick! I knew him well' which should not have the word 'well' in there. i.e. it should read:
'Alas poor Yorick! I knew him, Horatio: a fellow of infinite jest, of most excellent fancy' etc, etc
Running a close second, would probably be the one from Casablanca
'Play it again, Sam' which again is incorrect, Bogart never actually saying that exact line.
Chock