The main yuck factor....water that drippled from the used cups into other cups once the person had drank from them as they tossed the mostly empty cups into the center disposal hole.
Sure. Teachers used to pack the paper cups so tightly into the round tray holders then pour them up to the brim. Instant disaster with kids and older members trying to pry them out!
One time, as a 12 year old deacon, I was following a smaller kid too closely and I plowed into him with the water tray. He not only got about 10 cups of water spilled on his back, he fell forward and dumped his tray on the floor. A huge mess resulted because his tray was mostly loaded because he was the coveted assignment that only served members on the back pews and the side foyers. We were a bunch of bratty immature kids that found the situation downright hilarious. A bunch of know-it-all elders and high priests got up to restore order. They began to strong-arm us and pinch our arms as a way to sober us up. I had an annoying smirk that caused the other boys to crack up. So I was marched up to the stand to sit next to one of the BP counselors and a grumpy HP. I continued to create mayhem while sitting on the stand. I would stare at my cohorts then I would make funny faces and arm gestures until I would get them laughing.
The following Sunday, we were forced to practice being reverent, walking and the passing of trays. The YM prez and his assistant began sitting and passing the SM for a few weeks in order to coerce us to behave.
There was a time that wards were forced to use hard plastic water cups. Oh those were terrible! They were so noisy and the entire Aaronic priesthood enslaved in SM administration made it a point to really crumple up those indestructible plastic cups. SM became so noisy that we were singled out for making SM so irreverent and driving away the spirit. Never mind the endless crying babies or the cough-a-rama from the older members.
I saw someone drop a communion wafer is a Catholic Church...no big deal. They just picked it up. Maybe in the Middle Ages it would have been a sacrilege or something.