There is a particularly aggravating variation of the escort mission I call "defend your backup." Basically, there's this old PS2 era game called Battle Engine Aquila that makes a great deal of the worst offenses possible in terms of hated game mechanics. In a particularly egregious example, some of the late game missions have you assaulting multiple enemy strongholds (in a time limit no less) which is hard enough to while keeping yourself alive, and in the middle of your attack, your allies send in "backup" who immediately draw out the enemy and force you to go handle them before your teammates die. Escort missions are bad enough, escort missions under the pretense that they should be augmenting your assault, and not distracting you from it, to the point where things would be universally better if they had not been sent at all, induce controlled destroying rage.
In line with this is the "mechanic" of having a vast disparity between story elements and functions of gameplay, especially when it's clear that this isn't simply due to game engine inadequacy, but seeming laziness or disregard on the part of the designers. BEA again has a frustrating habit of presenting the player with a whole base of military elements to defend, all armed with weapons, and fully in knowledge of an enemy attack. The enemy lands on multiple fronts, the defensive units open with a single volley of return fire... and then... nothing. The whole base just sits there and gets annihilated while you have to run around like crazy and deal with each force on your own. My friend and I played the (depressingly few, which is to say, three) co-op levels religiously, since the added dynamic of a teammate went to great lengths to mitigate the overwhelming frustration of being the only soldier trying to defend multiple waves of enemies, and there's one level where you defend a static base (I make this distinction because the other two levels have you aiding an aerial or water bound convoy) and we very diligently watched as the impressive-looking energy mortars would fire one shot each and then go dormant as if to say, "well look, we spent all of our funding buying those fancy experimental warbots and we kind of scrimped on ammo, okay?"
And it's not like this is strictly an AI problem, because there's actually one ship (it's basically a flying aircraft carrier and, importantly refuel-and-rearm station) that has a galley of anti-air cannons that it uses to great effect at close range, and obviously the enemy AI functions, it just seems like the designers decided, "if the player's team is actually as impressive as it looks the game would be too easy," and opted to simply have them do almost nothing rather than intelligently scaling their ability. This is especially frustrating because the game is so damn promising, it has so many cool features: switching between air and ground combat on the fly, massive levels with interesting approaches to enemy design and ways to fight them (you can literally land on enemy battleships, knock out all of their close range guns, and pummel your weapons into the deck, then fly off when it dies,) epic scenarios (like hopping between multiple flying ships as you front an assault on an enemy base, raining bombs and massive energy blasts down with impunity, then turning around and taking on enemies from above, or, seeing a landing party coming in, being able to shoot down the enormous transport planes and take out the attachment of troops and armor the carry before they can land and deploy, giving a sense of urgency and demand with out the heavy-handedness of a simply countdown (not that the game was above tarnishing those as well,)) and genuinely interesting and innovative gameplay, and then it makes it annoying in most cases to play. Which leads me to...
Novel or enjoyable gameplay that is functionally not actually possible to execute within the game. The biggest offender I can think of for this is Deus Ex Invisible War. The game is generally panned by critics, especially compared to the original, but what I find most offensive is not the blatantly apparent downfalls in conceptual design (which can mostly be lumped into the idea of "dumbing down" elements of the original,) but the potential of mechanics which exist, but either are never able to be realized in the course of gameplay, or requires situations which have to be so neatly engineered by the player that it's hard to justify them as more than a chore. For example, the player can get the ability to take over enemy robots by "activating" them point blank range, but the environments are so hemmed in and most of the more powerful robots (for which this ability would be the most interesting to use against) are so entrenched behind the enemy troops that said robot would be used to take out, that you often have to dispatch all hostile elements just to safely get to the 'bot, thus rendering the whole advantage of being able take it over usually moot. Further, a really cool feature that has no ability to be utilized is the fact that grenade "items" when dropped on the floor as collectibles can be shot at to be detonated remotely (thus allowing the player to use them as impromptu traps in lieu of of, or in conjunction with, proximity bombs, which can drastically increase the potential for destruction with a limited set of tools.) With the aforementioned confinement of space, especially compared to the original, this is impossible because anywhere you would be able to set up a trap that could actually be useful, you'd have to take out the very enemies that the trap would be useful against. Another example of this kind of flaw is a cool late-game SMG that shoots deployable mini-bots as a secondary fire, replacing the standard SMG's flashbang launcher, while not requiring you to find the normal grenade version of the mini-bots and not having to take up a second space in your inventory for them. Unfortunately, by the time in the game you can get it, it's pretty well useless on nearly all of the enemies you face, thus eliminating any versatility or fun-fact that could have been extracted from the idea.
I guess these are a lot more game design problems than mechanics, but still...
