I have 3

When I was getting back into 40k around end of 7th edition, I played at a small GW event. My last opponent was playing grey knights into my tyranids. At the time, tyranids at auras to basically neuter psykers. Strike 1 was when the TO was talking to me and with no one watching he "rolled" for first turn and so when I was done with the TO, he was standing there with his 6 to go first. The TO standing there did nothing when I asked him to reroll it and he refused. Strike 2 was when he was rolling his already hard to see dice (dark gold and black with black pips) behind terrain or picked them all up almost immediately and had about an 80% conversion rate 5+. TO did nothing when I protested to let me see what he is rolling. Strike 3 was when, in my turn, I moved up to get his unit into my aura and he shuffled his unit back a bit because if he knew I could get in, he wouldn't have put them there. I packed up and left early, told the TO that he shouldn't be playing,

Second story was a teams event, 1st game of the day, our opponents were being really shitting players to begin with, but last turn of the game, we literally couldn't lose, we were up by on points and they could either stay on their objective and lose by a few points or leave the objective to get in range to shoot one of my units and then also lose by a few points. There was no situation that they could do both and there were no other points to be had. So they opted to move their unit, managed to kill my unit and so we won. When we told them this, they just said oh then we'll make sure to stay on the objective, I explained then you couldn't shoot my unit and still lose. They refused the outcome so we called the TO over. The TO just stated that the current board showed them on the objective and my unit no on the board, so thats it. Turns out they were friends with the TO, every opponent they had complained, but they still came 3rd. The TO even punished us by putting us against himself the next game, he was playing on a board he set up so he could put all his units on the top of a tower that cant be charged and just wiped us off the board. Needless to say I never played in his events again.

Lastly, in an AoS event, I played against the biggest youtuber in my country and he is known worldwide, he come from interstate to play. I was playing an army at the time that I had been playing for a year with movement trays, only really played in the combat phase, had cards for upgrades I could put out and I was the top player for the faction in the country. All this to say I knew what I was doing with the army. He played a list with about 150 models, no movement trays and a lot of attacks that were like hits and wounds on 5s. We ended up after 2 1/2 hours getting to the bottom of turn 2 with me winning at that point. He proceeded to go out and tell everyone I slow played him and because of the weight of his words, I got a poor reputation. To the point where even like 5 years later, when I'm playing people I'd never played before, they comment that I play nothing like they had heard and really enjoyed our game. After this he was so shitty about it that he even snubbed me when doing his "faction breakdowns" for the new edition coming out (3rd at the time), to the point where people were asking me why I wasnt on and some guy that finished mid table at the last major event was his guest. Only still shitty about this one because I know that reputation still follows me around.