Games where the enemy cheat, breaking the game rules.
I dont mind a bit of cheating or better initial positioning to keep things hard and interesting. I hate when you and the enemy figth on different rules, like I can lose subsistem but the enemy is at 100% until dead
Good old Civ4, where the AI will *always* keep up to your tech level, even when you are cheating. Whats the point in trying to manage everything as efficiently as possible when it doesnt make any difference?
the Terran AI never lifts its buildings off
Most RTS AI can't garrison units either, as another example.
I remember in Age of Empires 1, the A.I could get fouled up in it's build routine if there was water or a tree in the way of the locations it placed buildings. Also in Pokémon Red and Blue, the opponent always uses a move that is type super effective against you even if it doesn't do any damage, like constantly using Light Screen against a fighting type.
Hm, what else. They couldn't use transports in AoE 1 either and would get stuck if their villages kept getting killed by wild animals because they'd run out of food. Not to mention placing their town centre right where they begin in Nomad regardless of available resources.
In Dune II the AI only ever attacks your base from whatever direction their base is at. It's probably excusable, given that that game is like a thousand years old.
On that topic, the overarching problem:
Modern RTS where the AI sucks dick even though the rest of the game is awesome
No matter how competitively balanced the multiplayer is, or how creative the campaign is, or how cool it looks, or how intuitive the controls are, modern RTSs are all ridiculously bad at AI.
It's inexcusable for two reasons: a) if most games' developers put the effort into the AI that they do for everything else, the AI would lack most of the problems mentioned, and b) if the effort put into RTS AI resembled the effort put into the AI of many other genres, it would similarly be free of most common problems.
Obviously it's gotten better (see the Dune II example, Starcraft II being a step up from SCI, and so on), but there's still a huge amount of common-sense fixes to be done.
See GalCiv 2. It may have been mediocre in many ways, but there are settings where it's acknowledged the AI will get more resources/etc than you, but even ground with the thing at full intelligence? It will outplay you and plan right up your ass. Amazing AI.