I remembered this the other day, a conversation about banks and deposit slips, and I remembered how when I was young a friend of mine and I would pop over to the local bank and while making our own withdrawals or deposits would fill in the back of the deposit slips with notes like "This is a stickup. This is not a joke. Put the money into a bag..." or some such, return the deposit slips to the unused pile, and laugh and joke and imagine what would happen if somehow one suddenly slipped past, a customer didn't check the back of a slip...
...and one day it did happen, our bank, surrounded, they determined it to be a "hoax" but we got wise, stopped doing it, it was a lot less funny when there was a chance someone might get hurt...