Mercule said:DMs who change the rules midstream because an action I just took (by the rules as written) is "unrealistic", but only if I succeed.
DMs who change rules to hinder tactics that the PCs are having success with. (Like neutering Sneak Attack/Flanking mechanics when we've got two Rogues and no Fighters in the group.)
Anytime a (non-deity) NPC does something that the PCs could not ever do, regardless of what level, spells, or abilities we learn.
DM's who are having a hard time running high-level combat, so they assume it is that the PCs are "too" powerful or that the CR for the creatures are all out-of-whack, then start making comments like "Epic Monsters coming right up!" when your group is only around 13-14th level.
DMs whom you tell a few levels in advance, I'm going to get this PrC, is it OK with you? They say yes, then when you reach the level to get it, they say "You know what, I changed my mind, I don't like it." Now what do you do with all those feats and skills you used to get that class?
DMs who constantly say, that rule or class ability is so broken, but don't worry they're gonna fix it when the next book comes out. This is usually in regards to a class ability or rule that in the opinion of the group has nothing wrong with it, the DM just thinks there is.
And after the above, the DM goes and gives "unbalancing" things to NPCs because he thinks its cool and unique.
I know it's the DM's world and game, but...BLEH