Okay, strap in for a weird one. In middle school I had a group of friends that liked to screw around in a lot of various ways. We had a bunch of nonsensical inside jokes that at the time, we thought were hilarious. One of them involved shrugging and saying "good times". (This was said in various contexts. e.g. when a teacher was telling us to stop screwing around, or when we saw each other in the hallway, etc. etc.)
Another stupid joke was raising our hand in class backwards with our palms facing towards us. It just looked weird (and kind of regal) and we thought it was brilliant. (I think that might have been from Billy Madison? If anyone knows, please remind me)
Anyways, flash forward a couple of months. We've been engaging in these dumb jokes regularly. There was a really sweet mentally challenged girl in our class, and she was all about remembering things about people/saying hello whenever possible. Well, she took notice of some of our inside jokes and kinda horned in on them a little bit, but we didn't mind! It was rather endearing that she would do the special "backwards hand wave" when she saw us in the halls. We would backwards hand wave to her too because, why be exclusive about shit like that? It made her smile and it made us smile in such a way that we were laughing together, not at her or anything like that.
The problem began when she also started incorporating "Good times!" into the hand wave. As the days went on, she got more and more lazy with the arm/hand, which gradually went from a fully extended-up arm, to a more relaxed motion in which the arm was bent at the elbow and the palm was not necessarily fully open.
You might have guessed where this is going. I can only speak for myself, but I'm pretty sure that NONE of my friends realized what this was starting to look like. But the teachers/administration sure as hell did. From their perspective, this group of shitty little 8th grade boys taught a mentally challenged girl to do a jerking-off motion while saying "good times". Definitely the MOST fucked up thing I've ever been accused of. Needless to say, we got suspended after a traumatizing interrogation style sit-down with the school security guard in which he went around the room forcing us all to say "masturbation" one at a time, and not allowing us a chance whatsoever to explain ourselves.
TL;DR: Mentally challenged girl picked up on my middle school friends' and my inside jokes (of her own volition). Administration thought we intentionally taught her to mime jerking off while saying "good times".