Poll Results

Total votes: 548

50% sure or less: 136 (24.8%)
60% sure: 60 (10.9%)
70% sure: 60 (10.9%)
80% sure: 72 (13.1%)
90% sure: 57 (10.4%)
95% sure: 38 (6.9%)
99% sure: 20 (3.6%)
100% sure: 75 (13.7%)
Even if I was 100% sure I wouldn't give it back: 30 (5.5%)

This one generated a lot of responses that I didn't expect. Many people wrote by email and on the forums to say that this situation would never arise because they never or almost never possess or use $50/£50/50€ notes. That didn't occur to me, because here in Australia, the $50 note is the most used banknote. ATMs dispense $50s by default, unless you specify a withdrawal total that doesn't divide evenly by 50.

I was inspired by an actual event, just a day before I updated to this poll question. I was buying lunch at a fast food place, and handed over some cash. The server handed back to me $38.50 in change. I stood there staring at it for a few seconds, thinking, "Wait a second. I gave him a $20. Not a $50. Didn't I? Wait... did I even have a $50 in my wallet today? I don't think I did."

Anyway, I was mostly sure that I had given him a $20, not a $50, but not 100% sure. I'd say I was about 80% sure.

What did I do? I spoke, up, said he'd made an error, and gave the extra $30 back.