On daf 74b of Kesubos, Rashi seems to be saying that when the husband comes to Beis Din to divorce his wife because she is a "nidranis" or "shem rah", the Beis Din must warn the husband that he will not be able to remarry her. This is to prevent mamzerim because if she gets married to another man after the get, and the ex-husband says "if I would have known that the neder could have been uprooted (or the shem rah was not true)" then he is mevatel the get, she will be an "eishes ish".
Tosefos disagrees on the spot saying that a man can not be mevatel a get "L'mafrei'ah".
Where does Rashi get the idea that after this husband freely (willingly) gave his wife a get (even without the warning of Beis Din) that he later can undo that get simply by saying "if I had known.....": How does that statement undo a get L'mafray'ah? And if it can - it seems that in certain circumstances, a husband can create terrible problems for an ex-wife if he so desires.
best regards
Jeff Ram,
Jerusalem
Rashi is speaking where he said clearly at the time of the Get that the reason that he is divorcing her is because of the Shem Ra or the Neder. If he didn't specify the reason, obviously he cannot regret afterwards. In this case, the Chachamim safegaurded against it by saying that no matter what, he cannot go back to her and therefore it is clear that his Get is unconditional. Tosefos dismisses this P'shat for he says that in order to make his Get conditional it needs an explicitly-stated Tenai.
Dov ZupniK