Probably not the worst of all time (that title goes to u/Spez, former moderator of r/jailbait), but I once made an innocuous post on r/DeathGrips, the community page for an industrial hip-hop group, saying that the shitposts were getting pretty unfunny and the overall quality of the sub was going down and spent the next ~10 months dealing with random PMs from the head mod insulting me and sending screenshots of himself downvoting every single comment/post I ever made (even ones outside of his sub and posts I made before that post). That was definitely a unique experience.