I've had that happen a couple times, too. The first time I was super excited, and looked up the collectable price, and it was like $8 for a (pristine) $5 bill. I think I kept it for a few days to show to people, and then spent it. I inherited a couple from my dad last year, and the collectors' price hadn't changed, so I did the same thing. Still cool, though. I hope whatever cashier received them from me got a similar thrill.