About 12 years ago, I was on a bus and there was a girl about 14 years old who was crying her eyes out. I talked to her to find out what the matter was and it turned out she'd saved up £15 for her mum's birthday and had lost it.
She was so upset I decided to give her the money and also gave her my business card (I had that sort of job in those days) and told her that maybe one day when she was older she could send me the money back.
Obviously, I never expected it and had totally forgotten about it... until got the post today. My then workplace had forwarded on a letter to me which they'd opened and decided I should have.
It had a £20 note and a letter that just said "I'm the girl who was upset on the bus and your kindly gave £15 to so I could buy my mum a birthday present. Here is is back with interest, sorry it took so long but I lost your card and then found it in an old box of stuff."
Absolutely astonished and actually found myself having a little cry for some stupid reason.