When we were kids a friend had what I now believe was a bsa cadet. It was rubbish, took maybe 4 cocking and firing cycles for the pellet to come out the barrel. We were messing about, cocking and dry firing it. I pointed it at him across the room and a pellet came out! Hit him in the forehead square between the eyes. Still had enough power to leave a small white lump and caused enough pain to think he was going to smash me in the face.
Not really a regret though.

Only regret was selling my webley senior 5 or 6 years ago. Why!!! I didn't need the money and I loved that pistol. The moment the buyer left with it I realised I'd been stupid.