To get back at some elders in the next room for a prank they pulled on us, I got 8 elders. We each took our travel alarm clocks and set the alarms to go off on the hour starting at 11pm (with the extra one going off on the half hour sometime in the night.) We hid the clocks all over their room including in some locked drawers. We went to bed chuckling. The next morning at 7am, those guys still weren't out of their room. We went and knocked on the door. One came to the door and he had black circles around his eyes. He handed us our clocks (well, most of them anyway) and shut the door again. They didn't talk to us for the whole day, and a year out on our missions, they still bristled when we would mention it. They never did laugh about it. Not even 2 years later or a mission reunions. Not once.
We hadn't realized the head games it would play. They said when the first one went off they tought it was funny. The second one not so much. The third one and they spent the next hour trying to find more clocks. They thought they had them all, but didn't know about the ones in the locked drawers they couldn't reach. Knowing that a clock would go off each hour prevented them from going back to sleep because they would fixate on it. They hardly slept at all tat night and it is tough to make up sleep in the MTC.
Looking back, I still think it was hilarious, but I also feel pretty bad too.