1. As a healer main, I know that if I have regen on a tank and they pull, enemies will aggro on me. However, some tanks only pull on group at a time, others pull multiple groups. So, I see a tank pull one group, they stay there for a few seconds so I think they are only pulling one...so I cast regen, and then they run off. Or, they have regen and leave one enemy of the group alive then pull the next group and I get aggro.

2. Healers that do not heal. I have run dungeons where the healer(normally scholar) decided the dungeon is "too easy" and starts pulling and DPSing everything and people die.

3. Speed running dungeons when there are people new to the dungeon. As a completionist, I want to get the full map achievement. Many times, I ask if we can get the full map, and they refuse, speed running the whole thing. I think if someone specifically asks for it, you should help them get it. That is why I ask if there are new people "do you want the full dungeon?"