First gamer who just annoyed the hell out of you.
I'm not sure which one was first the first to do it, but my youngest two siblings and a cousin of about their age did their level best for a while to push my buttons. At first, it was just an obsession with bar fights. I had run an adventure which began in a tavern, and some random NPC had offended them (by acting indifferent toward their characters rather than treat them as the Big Deal that they clearly were in the minds of their players) to the point that they decided there was no alternative but to seek revenge. A huge brawl ensued. I guess they enjoyed it a little too much. For months after that, they didn't want to do regular adventures at all. I'd drop a hook or two, they'd ignore them, and say, "We look for a bar." Sigh.
Then there was their "shock" phase, when they'd say their characters were doing all sorts of revolting things, mostly involving bodily waste and NPCs who weren't subservient enough to suit their preteen egos, to see just to what extent I'd really let them try whatever they wanted to do in-game. If I had been a meaner DM, I probably would have made one of those "uppity" NPCs a powerful warrior or spell-caster and put that batch of player characters in shallow graves, or serving out a fate even worse than death. As it was, I remember just walking away from the game table in frustration quite a few times.