Assumption that the Lender will check out the witnesses' vailidity: Ketuvot 18b
Assumption that minors wouldn't sign a Bill: Ketuvot 18b-19a
As though their testimony has been verified in court: Ketuvot 18b, Gittin 3a
Having a Kuti witness: Gittin 9a, 10a-11b
A witness who can't read, or sign: Gittin 9b, 19a-b
A witness who can sign, but can't read the bill: Gittin 19b