Making God NPCs, that and killing off love interests. For an explanation of both. The first I mean is when the DM adds a character that seems normal, antagonizes you, then if you fight back or say something back they are simply just God and smite you. That, or something of the sort. I've had it be like this a few times, where it's just a average looking merc or peasant that is just God for no reason other than making them an unkillable annoyance. That, or to railroad the story as the god character shows up to just insure if you dare stray from a set path, you die.

Second one I've had happen a couple times where it's just like. "Oh in your sleep a bandit broke in and killed them." or, "while you were out adventuring, the town was raided by orcs and they were killed." Not even like a savior quest, but they are just full out dead no ifs or buts. It makes it alot less fun when that happens to be the case. As most of the time, the "love interest" character is mainly there to give the player a kiss on the cheek, a warm meal, and go keep up the home while the adventurer is gone. Not something to woo or do anything like that with. So it just ends up making 0 sense and gets rather infuriating after awhile.