Happens all.the.time!
I was stuck on HM one time, couple years ago.
It wasn't due to stupid guest tricks but got to witness a stupid guest trick during the outage.
We were right there just before you go down the hall with the doors banging, where the wind blows and they say something about "hot and cold running chills"
There was a young couple in the doombuggy in front of us and after we'd sat there for oh, 20 min or so, she jumped out and did a hand stand in front of that fake hall where the chill blows and her companion took her photo.
I don't actually know if the staff noticed her or not. They may have been so wrapped up in what was actually causing the outage and preparing to evacuating us from the ride they missed it. No announcement was made, no one came by to say anything to her then and when they did come to get us, no one said anything while I was in ear shot.
I guess she ended up with a cool pic