Thurs Gitten 4

The gemara says that R' elazar would Pasul a Get signed not L'Shma (although he holds a Get doesn't need to be signed in the first place) since R' Elazar agrees a Get is Pasul with forged witnesses.

Tosfs asks: how can we compare the two? He needs to pasul forged witnesses, for perhaps he cannot produce the handing-over witnesses, and Bais Din would rely on the signing witnesses to validate the document. However, this is not a problem if the witnesses sign not L'Shma. So why would R' Elazar Pasul?

Tosfos answers: by L'Shma, we have a different problem. we need to enact for teh witnesses to sign L'Shma, for if they don't, perhaps that may lead that they'll also write the Get not L'Shma.