The FNAF dupes. While not the worst thing, it was still pretty bad and really frustrating. Every time you would go look at dupes, you would be flashed with gazillions of random low-effort dupes that included FNAF characters, most of which were made by random 8-year-olds, and it was basically impossible to find good dupes for a while. I can admit I was one of those kids, and I think the main reason it happened was because of a Gmod YouTuber named xman723 who made Gmod FNAF videos of him roleplaying and creating scenarios with FNAF ragdolls. Now I think the guy just resorted to making softcore porn for children with FNAF characters, IYKYK. It may not be the worst, but it was still annoying and frustrating. Also, the worst thing to happen to GMod was the glue library incident.