Stopped off at an ATM at a bank near my oldest brother's house one weekend while I was visiting.
As usual, I check the money to be sure I got what I asked for from the machine. Flipping through all the $20 bills, I passed a $50 bill. WTF? Backed up...yep, that's a $50 bill! Checked the rest out and they were all $20 bills.
Parked the car and went into the bank. Lady in the lobby asked if she could help me. I said "Sure! You might want to secure the drive-up ATM and check all the money in it. I just withdrew $400 from the drive-up ATM and it gave me a wrong bill. I'd like to see if I could exchange the other 19 $20 bills into $50 bills to match!"
That got their attention!
They exchanged my $50 for a $20 and secured that ATM so they could pull the money and check it. Lady was surprised that I came in. I told her that I'm sure every penny is accounted for in any bank and it wouldn't be right for me to place somebody's job here in jeopardy over something silly like this...I make more than enough, I don't need to be dishonest.
As it was, she said that there was a slight imbalance in the books. I don't remember the amount she quoted me, but that $30 difference actually went a long way towards balancing it.
In all the years I've used ATMs, that has been the ONLY time I ever got an incorrect amount on a withdrawal.