Pretty much every kind of fake difficulty bothers me to some extent, but I think when a game betrays the player somehow is what ruins it the most. Usually this is by denying me information (this item is critical to progress, but you missed it and now you're in a zombie situation) but sometimes a badly designed game can do it just by virtue (such as having controls so bad that you can't escape, say, a chainsaw attack, because the controls didn't register or your character turns too slowly or whatever.
Now, it's perfectly fine to betray the player if you have a good reason for it, like BioShock did. If not knowing some item is critical to progress and going on without it into the aforementioned zombie situation IS a pivotal aspect of the game's story or experience, good. If not knowing some item is critical just because the developers forgot to put in a fail-safe or a warning or just decided to be assholes, then not good. (Kind of like in Star Control 2 on the 3DO where some very important dialogue concerning the game's TIME LIMIT just got left the hell out)
I tend to rail on horror games a lot because they tend to have bad controls or some other stupid-ass or broken game mechanic, which is then defended as "making the game scarier". Dead Rising didn't let you save anywhere because it "added to the tension". As far as I'm concerned, bad game mechanics should NEVER be defended in such a way. Bad design is bad design, pure and simple. There are ways to make games scarier without saying "You don't get a quick-turn-around button, assholes". Silent Hill has a bad combat system, and people just say "Oh, well, your character's a regular joe so he's not good at fighting." Then, I don't know, don't let me fight at all like Shattered Memories. You don't have to give me a flail-around-and-cry button to prove the point that the character's not a fighter.
This is an ancient example, but Jet Set Willy had an absolutely awful bug where entering one room would, through some bizarre turn of events, completely and totally corrupt the game. And the developers said oh, well that room's full of poison gas. Poison gas that makes the game not only unplayable, but unplayable forever? Please.
One last thing, since I'm on a roll, is enemies with unnatural resources, like said above. In Champions Online, there's no difference between a level 1 grunt and a level 40 grunt other than the level 40 one has more health. I gotta say, this game doesn't make me feel like a superhero when it takes me ten minutes to slowly pound on a HENCHMAN just because he has an unnatural amount of HP. Here's what designers should do: introduce NEW enemies, instead of just strengthening old ones. Maybe it starts with a Rifleman, who has a rifle. Then comes a Stormtrooper who has a machine gun, and a Rocketeer who has a rocket launcher. Then comes a Gatling Gunner. They all have the same amount of HP, but they all have different tactics and ways to kill you. Difficulty by strategy, not attrition.