The Clapper
At the fast food place I worked at I had a running one where the light switches for the bathrooms were in the main dining room. So everytime a new person would start I would tell them the lights were clappers. They would go in there and clap to try it out and I would just flick them off and on.
Usually they would catch on after 2-3 shifts though. Well we had one person who I was able to have her believe it for about a month and a half.
She would even clap and I would be away from the switches and would come out of the bathroom to ask if the clappers were broken and I would have her try it again and just flickered the lights. I was a little bored there sometimes. Story credit: Reddit / thenome