I've had a few.
On my second shift at the hospital gig, I tried a simple trick for one kid who was getting ready to leave. He literally had nearly a dozen family members jammed into the room. They started talking in an Eastern European language among themselves - I later discovered with the intent of ruining my tricks.
First time I tried the trick, the kid said I didn't get his card. OK, maybe I screwed up - I was still green (and technically still am, though I'm more polished now). I tried it again, and this time, I knew I had the right card, because the kid actually gave himself away with a facial expression when it became visible at an earlier stage of the trick. But he claimed again that it was the wrong card.
Now I decided I was nailing this trick, no matter what. I tucked away the deck I had and pulled out a stripper, inserting the kid's drawn card as you're supposed to for easy discovery. Even better, as I was shuffling the deck, I kept asking him if he's completely sure he's remembered his card - cutting to it and flashing it to him just in case he and his family forgot. This time they knew I had them cold, so they stopped screwing with me and admitted I had their card.
I went to try another trick - I usually go for a three-trick routine. I was doing the original WOW trick, and part of the routine involves intentionally drawing the incorrect card on the first go. I drew the wrong card out, asked if it was his card - and the little wiseass said yes, killing the trick right there. At that point, I mentally said "|=uc|< you very much", grabbed my gear and bailed. Which goes to show that some people still get their jollies trying to screw around with their fellow man.
So, that was the worst. Now here's the funniest.
I was performing the "insurance policy" gag on a little nine-year-old girl at a party. The key to the trick is to force a card, then screw up a few times in a row trying to find it in the deck. So I force the ten of diamonds to the kid, shuffle up the deck really nice and thorough, no attempts to control any cards, make a cut to show her the wrong card but without peeking at it first - and wouldn't you know, I cut to the ten of diamonds... World's shortest "magic" trick.