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daf bit: Gittin 20
The mishna teaches that a get (bill of divorce) can be written
on almost anything, including an olive leaf, the horn of an ox,
or the hand of a slave. (Rabbi Yosi says not on living creatures
or foodstuffs, which the rabbis confirm in the g'mara.)
The man must give his wife the item on which the get is written;
the rabbis teach that if he says "here is your get, but the
sheet belongs to me" she is not divorced. However, he still
has an out: if he says "here is your get on the condition that
you return the sheet to me" then she is divorced;
this was a conditional gift, which is still a gift. (19a (mishna),
20b (g'mara))

no subject
Halakhic court procedure puts pretty much everything on testimony of witnesses. E.g. if you buy a house and live in it continuously for a certain number of years (seven?), and you know you can bring witnesses that will attest to your residence and ownership, then you can destroy the bill of sale.
And in the case of a get, you need witnesses no matter what, because someone needs to confirm that the document was prepared and delivered in the right way.