Shuffling along in line for the cash machine, trying not to peer at everyone else's balance, I get to the front and notice the last man at the helm has wandered off mid-transaction, leaving his card in the machine, and me with the option of a free £200.
What to do, what to do? My long history of shop-lifting and theft from employers fought against all the moral lessons I've ever been taught, and eventually my natural impulses were overcome. I removed the card and followed a stranger down the street until I could do him a good turn.
And was he grateful? Was he?
Well, yes, as it happens. He said "Oh, thank you. How kind." And I replied "It's my pleasure."
Then I came back to work, unable to account for my behaviour.