A client hadn't paid me in months. Luckily I had a contract:
Me: If you don't pay immediately you're going to start incurring late fees.
Client: All right! All right! Jeez. I'm putting it in the mail right now.
It didn't dawn on me until later that all our transactions had been online to that point. Three days later I received a check - postdated three months later.
Jokes on him, though - I deposited it in an ATM and the bank accepted it that day.