The guy was doing a trick where he wrote on a ping pong ball, gave it to me to look at, destroyed it and then made it reappear.

While I was looking at it I marked it with my thumbnail.

When it reappeared he asked me to verify that it was the same, but it didn't have the nail-mark on it, so I told him that it definitely wasn't, and how I knew.

I was 12 and my parents were furious with me about it.

For some reason they never ever took me to a magic show again.

I want to believe that a better magician would have noticed and reproduced the mark.

Heh, I could probably talk to a shrink for several days solid about that.

In their view at the time I ruined it for everyone and wasted the money of the entire audience and possibly ruined the career of the poor magician too and if I had any kind of feeling for others I would have played along and just lied. It was socially unacceptable behaviour, apparently. I found the whole thing completely bewildering.