I did a small project for a Sun photo shoot during the early part of their glory days when they were growing exponentially. I sent them an invoice for $2700, and was mailed a check for $27,000.
Because I was honest as well as (arguably) foolish, I called their attention to the mistake and never cashed the check. I occasionally wonder it they would have ever caught the mistake on their own. Given how out of control they seemed at the time, they quite likely would have never noticed, but at least I never lost any sleep over doing the “right thing.”