I've literally had it happen to me at a fairly large (~15k employees) company. I told my boss I was quitting, we agreed on a date for my final day, that day came, I said goodbye. And then I got two more pay-checks, before anyone noticed I was gone. Apparently my boss had completely forgotten to tell whoever he was supposed to tell that I'd quit. I had to pay the money back, but the company gave me a 6 month, interest free, payment plan to return the money.