Probably when I was at locals running scraps. I was set to duel this pretentious jerkass who was using Chaos Dragons (back when they were tier 1). First duel, I end up Solemning/Bottomlessing everything he tries to summon, and I win. He whines and says that the duel didn't count.
Second duel, I got a nice lead, but then he manages to fill up his field and banish my Scrap Dragons with BLS and Chaos Sorc. He attacks, and I activate Mirror Force. He shouts profanities, slams his fists on the table, and calls a judge.
He tells said judge that I was stacking my deck. The judge rifles through my deck and checks my sleeves, then informs the guy that I wasn't cheating. Guy swears at the judge, scoops up his stuff, and stomps out of the store.
I ended up losing in the top 8, though.