It has to be a case where the AI player is doing something legit i.e. the computer is playing by the rules and is fully within its rights to do what it does. So exclude stuff like glitches or advantages that developers give the AI to compensate for real intelligence and increase difficulty (like allowing AI to "see" the whole map or give it unlimited resources).

Here's something to start:

_Heroes fleeing the battle in HoMM3. Nothing, absolutely nothing, is more annoying in that game than seeing that. And they do it so often. Especially after a long battle.
(Conversely, nothing makes you more happy than for example seeing that high morale that allows you to kill that last enemy that you know would have been used to flee the next turn).

I hate it when the AI tries hard to survive in strategy games (sometimes beyond what a human attempting the same could reasonably achieve for real-time strategy games) when it is completely hopeless and prolongs the inevitable for what can be quite a while at times.

It's cool to see things reach their natural conclusion the first couple of times, but after you're at your 100th game, you'd kind of wish it would just keel over once the outcome is patently obvious and let you move on (not fun to keep playing after the challenge is gone) like a human player would.

Starcraft pops to mind, where the AI would hide at the remotest corner of the map trying to rebuilt it's destroyed base, but there are other offenders.

Fixed.........Shackles of War.

I'm more of a HoMM4 man myself, and it was sooooo annoying to have to chase down AI armies around the map, especially the ones that kept raiding my garrisons and supply routes but avoided my main army. It's totally legit, they would run away from any army strong enough to wipe them out, but it was sooooo annoying.
I still remember one map in particular, a large costal one, with two large interconnected areas of ocean and many many inlets. I had no costal cities, which meant no ships for me, so turn after turn I was getting massacred by marauding armies, only to find absolutely nothing when my main army got there. Even when I got lucky enough to find an abandoned enemy ship and started patrolling the seas with it I couldn't find them anywhere. That was the key to winning though, since I started using it for sea denial tactics. =) With the reduced pressure on my supply lines, I was finally able to push foward over land with my main army. Eventually I captured several other abandoned ships and gained control over the seas, only to win without fighting a single sea battle. My guess is that the AI saw them coming and disembarked its armies. Cowards. =) But again, totally believable.
Another example are enemies flanking you and taking cover (and shooting while in cover) in FPS. I've seen it done poorly over the years, but one game comes to mind where it was done right: FEAR. The AI in that one was very well done indeed: flanking, taking cover, creating cover by knocking stuff over, firing from cover, using grenades correctly, calling for reinforcements, even pulling back when things were going badly. Good thing they were so chatty and kept revealing their intentions. =) Still, it was annoying how consistently they would pin me down and flank me. Or maybe I just suck at FPS, idk...
And of course, spamming low body attacks in fighting games. Dhalsim in Street Fighter 2, anyone? Arghhhhhhh...

That's one nice thing about the first 2 (and maybe the other) Master of Orion games: you're kicking butt and have it won except for the mop-up, so you ask for a vote and let your overwhelming superiority settle matters at the ballot box.

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Been playing a ton of Mount and Blade: Warband, Prophecy of Pendor mod (fantastic). Generally kicking butt on the battlefield but every once in a while I get hit by the Magic BB - someone's well-aimed javelin, etc., gets in a good hit and knocks me out in one shot. Or takes my horse out from under me when there are no friendly troops around to bail me out. Dagnabbit!

Yeah, they did a good job with the MOO games for that. Also, in MOO2, you can go beat the Antarans too, though you got to use it responsibly when you know the win is in the bag, because at a certain point in the game, any of the top 3 players could go beat the Antarans (the AI simply never does).

Here's one:

In a Dragon Quest game, getting ambushed by a Metal Slime (or one of its more powerful cousins), and the Metal Slime decides to run away before you even get a chance to enter a command. (For those not familiar with the series, Metal slimes are enemies with low HP, high defense, and a tendency to run away, but if you get lucky enough to kill one, they yield a lot of experience.)

Interestingly enough, starting in Dragon Quest 8, the developers made it so that enemies can't run away during the surprise round when they ambush you (yet you can still run if you ambush the enemies; sometimes it makes sense to give the player an advantage that the enemy AI doesn't get).

I could also mention the Sacrifice spell in Dragon Warrior/Quest 2; it isn't that useful for the player (since it kills the caster, doesn't work on bosses, and denies you the experience from killing the enemies), but is downright unfair when used by an enemy (if it happens, it's a guaranteed party wipe).

Another one (from a completely different series):
In Disgaea (original PS2 version, which has no option for turning off animations), when enemies repeatedly use Hell Pepper on your characters. The animation is annoying when you have to watch it repeatedly during the enemy phase before the turn ends and you get to actually do something. (I could put any enemy attack with a lengthy animation in this category.)

This reminds me of playing chess, against AI (or certain real opponents). You take their Queen early on, and they keep going all the way to the point where it's just their King hopping one space out of check over and again.

Save us all 30 minutes and call game already. Jeez.


Here's one: Enemies that are allowed to Kamakaze, like the [url=http://finalfantasy.wikia.com/wiki/Bomb_(Tactics)]Bomb[/url] in Final Fantasy Tactics. One minute you're kicking their ass, and they have like 10HP left, then on their turn they move right in the middle of your group and self-destruct causing massive damage to several of your characters. That used to piss me off so bad.

That's a relatively mild variation of the Dragon Warrior 2 example I posted; in your example, it's just damage, but in mine, it's instant game over.

I thought of another:
In Elminage Gothic, enemies can cast the Miracle spell, and can use it to teleport your party away. This ends the battle, and will send you to a random square on the floor. At best, it might take a moment (or a map spell/item) to get your bearings, and you might end up on a different branch of the level. At worst (and this has happened to me more than once), your party is teleported into solid rock; at this point, either you reload from your last save (recommended, even if you normally like to play Ironman style), or your party is gone for good (there is no way to recover your party if this happens). Do you think that's fair? (I note that you can cast Miracle, but it will cost the caster one experience level.)

Like everything I think up is more AI cheating than anything...

Uh, suppose RPGs and the like where you have one main character that cannot die/faint no matter what or game over... Swear the AI in those games always decides to gangbang that character and that character alone.

I'll never forget the AI in [url=https://en.wikipedia.org/wiki/Rise_of_Nations:_Rise_of_Legends]Rise of Legends[/url].

When playing the campaign you could select 3 levels of AI difficulties:

Easy. The AI was braindead. It never attacked you, it never built additional units and it never replaced its units that died. The game was so boring that it became unplayable.

Normal. I could only categorize this as very easy. The AI would occasionally send waves of 4-5 soldiers or 1-2 tanks and it rebuilt the units it lost. Unless you went away form the computer at the start of the mission, without building any turrets, it was impossible to lose.

Hard. The AI had INFINITE resources and it built units INSTANTANEOUSLY. I couldn't believe that the only way I could win any mission was by ignoring his units and targeting his production buildings. Because if I killed any of his tanks near his factories he would immediately spawn a new tank that would start firing at my units. On the latter missions unless I rushed and overwhelmed the AI in the first 10 minutes of the game, it would spread across the map and destroy me.

It was unbelievable! Had anybody actually tested this game? The game is such a disaster, set in such an intriguing universe.

No matter how well and skillfully you are playing, the opponent will be doggedly on your tail, not to challenge you, but to snatch victory from your hands if you make a single mistake; rubberbanding in a descriptive nutshell.

Or how about the AI not playing by human rules? IE: All of Final Fantasy Tactics.

When the aliens use Blaster Launcher against your squad in the original XCom game :P

When the AI builds a warehouse on one of your islands in the Anno games ]:-\

Ah, yes, I hated that in the first game... because you were not allowed to do the same, as they would instantly declare war.