Signing at night, where it was written during the day: Gittin 17a
Signing it a long time after it was written: Gittin 18a-b, 19a
The witnesses sign in the presence of each other: Gittin 10b
Validity of a Bill which contains an unnecessary signature, if that signature is invalid: Gittin 4a
Validity of Bill with an invalid witness signs first, if he wasn't officially a witness but only part of the writing of the Bill for the husband's personal reasons: Gittin 18b
Having a Nochri witness, where his name is recognizable as that of a Nochri: Gittin 10a, 11a
A Bill which comes from overseas, with witnesses signed on it whose names are names identifiable as names of Nochrim: Gittin 11b