Another one was when I got an address for a PO Box, a few boxes next to my own at the same Post Office. Really laughed that time while I was sticking the stamps on to hand it over while I was going to pick up my mail 🙂   I don't think the guys at the PO would have appreciated my going over with no stamps on the envelope saying "would you mind sticking it in box number 627?"

My local PO is happy to post mark any item that has correct postage on it and pop it straight into the recipients letter box.