The one that springs to mind is when I took some plastic straws out of a box, wiped my sweaty crack with them, then put them back in the box for people to use.
There are probably lots of others too.
I've also probably nearly made several people piss / shit themselves on trains a number of times. All members of the 'can't figure out that I should lock the toilet door to prevent potentially embarrassing incidents' brigade, so I have no remorse over those incidents. Every time the person shut the door from the inside, I was standing just round by the train door pressing the button to open it from the outside. My record was about six times, before the bloke gave up and went to use another toilet / shat himself instead. I wasn't bored during the journey and thought doing this would pass the time. I just happened to be getting off the train at the next stop and thought it would be funny.