What happens if you don't write the amount on a deposit slip?

I routinely do not write the amount on my deposit slips and I routinely do not endorse the check… when depositing into an ATM I have not yet had a problem. So as for “what happens”, my experience is that the deposit is completely without error.

It may be true that the deposit will work without a completed slip, however not filling it out yourself just shifts the burden to a bank employee. Everything will probably be fine if you forgot to do it once, but I wouldn't do it regularly.

Especially with cash. While the bank employee couldn’t probably make the deposit for less than the check without causing some sort of paper trail, it probably wouldn’t be difficult for them to reduce your deposit by $20.00 and slip a twenty into their sock or something.

I wonder if the ATM prints the amount you claimed
to deposit on the envelope as its fed through the machine.

Good question, Spargett. I wonder (but am not approving of) what happens if you “accidentally” put a sum $20.00 more on the envelope than is in the envelope. Do they contact you? Do they presume you messed up, or do they presume their employee messed up?