Games where the enemies can do things with basic, shared mechanics that you cannot.
For example, Dragonball Z Xenoverse. Everyone can block. But as a player, you generally accept that you can't start blocking in the middle of taking damage consecutively, like being comboed or being in the center of a beam attack. The AI, however, can. By what magical property this is so, whether its exploiting microsecond gaps in timing that a human can't, or because it's just been straight programmed to, it doesn't matter because it's still bullshit. Plenty of games often let harder enemies waive some of the core mechanics in favor of a difficulty spike. Guys not reacting to attacks so they can attack through your attacks is a popular one (DBZXV also does this.) I can accept that to a degree because it forces the player to adopt different tactics than what normally works. But what, exactly, is served by letting the enemy completely break the rules only to their increased survival and therefore dragging out the fight? If an enemy can ignore an attack because "reasons" and the player did nothing wrong, you're essentially telling the player they have to do the right thing, twice, randomly, because this guy is tough.
That's not an actual challenge in my mind, that's just attrition centered around the player's ability to execute. That it also is something the player would LOVE to do but can't because "reasons" is just kind of the slap in the face that makes it truly irritating. If you're going to let guys break the rules so they can be tough, please let them break new rules or do unexpected things, rather than letting them do something I can do but they can simply do much much better. Especially when it comes to something as core to the combat experience as timed blocks and so forth. If a guy wants vomit destructive rainbows across the whole screen, fine, let them, dealing with that is part of the challenge. But just letting them cut and run when I've got them, and by extension, every fucking thing I've fought in the game so far, dead to rights is the worst kind of difficulty because you can't do anything about it.