The tournament AI in Mount & Blade apparently decides that the best way to win is to drive into a wall. And then get fenced in by dismounted horses.
The AI will move up in sieges in a terribly disorganized yet straightforward fashion, causing them to often accidently push you, their commanding officer, off the ladder, which may result in serious fall damage, while they themselves are all too eager to get massacred at the choke point above, often without even getting a hit in. The best way to handle this is to go first.
Similarly, the AI in sieges and normal battles alike will waste their very powerful lords and cavalry units on a first charge of their own instead of keeping them in reserve to use them when they'd actually be able to inflict serious damage while your forces are clashing with the main army.
The famous Game Mod The Eagle And Radiant Cross, which puts the game in a Rennaissance setting, adding firearms and such, has a unit type which uses a hand cannon, a scripted weapon that has terrible accuracy, but can kill entire swarms of enemies with one well-placed shot, making it the bane of formations and tightly packed enemy hordes. The problem is that they will use this highly dangerous weapon as if it was a regular rifle. This means that not only will they miss out on chances to deal big damage, such as firing on a small group of enemies instead of a huge crowd, or even trying to unsuccessfully hit a single cavalryman circling them while a much bigger formation of swordfighters is closing in to turn them into dead meat; but they also will carelessly fire this weapon into their own ranks, often killing fellow allies in the battle or even the Cannoneer next to them because they had to shoot diagonally, if you're not careful, they will also shoot down you, again, their own commanding officer. It's quite telling that one of the most efficient tactics to use this weapon properly is to take a few of your Cannoneers into battle, then immediately execute one of them by shooting him in the head, then picking up his hand cannon and using it yourself to actually hit the enemy army where it really hurts.
One of the chief reasons why it's practically impossible to not kill any civilians by accident while actively avoiding contact in a ground vehicle in Prototype. It seems like once you induced panic in them, they'll abandon almost all of their reliance on their senses. That's also assuming you aren't convinced of their intention to commit suicide in the face of the game's setting.
Oh, Prototype. Thou art a shining example of this trope. Where civilians show absolutely no regard for their own safety, the most dangerous thing a Marine can face is another Marine and where highly-trained military personnel can TD Dance off of a platform to their deaths. It makes you a bit less guilty for killing them, because they genuinely come across as Too Dumb to Live
The slightest disruption in traffic flow can cause huge pile ups. Tanks will just drive over anything in the way.
Soldiers will charge in the direction of any loud sound that they hear - such as Alex landing on concrete hard enough to cause a small crater. However, even if Alex is the only one around and standing in the middle of the crater, they won't suspect him at all if he's in a military disguise. On top of that, if this is done close enough to water, it's quite likely that soldiers (who can't swim) will dive right off the island in their zeal to find Alex, crossing this trope with Too Dumb to Live.
Later in the game, you can accuse a random soldier of being you in a military base. Not only do the military always believe random accusations from a rank and file soldier, even if the same guy made the accusation 15 times already, but they always open fire. Even if they are holding a rocket launcher and standing next to the target in question.
Mercenaries 2: World in Flames is a crowning achievement in Artificial Stupidity. Frequently, enemy soldiers will run in front of vehicles, throw grenades at their own vehicles, crash said vehicles when at even the slightest deviation, attempt to plow their vehicles under your tank causing the game to assume the tank has run them over, drown themselves, run off of building ledges and generally kill themselves in a variety of amusing ways. This can be frustrating for numerous reasons, chief among them that Chinese RPG soldiers fire thermobaric rockets that do massive damage to you and the scenery. In fact, it is generally impossible to capture all the HVT's alive, because they will kill themselves or die at the hands of their subordinate troops.
The original was as bad, if not worse. Civilian vehicles would instant swerve into you even though you were in the other lane and in no way a threat (as far as they knew). Combatants would drive their jeeps into your tank, which was about as effective as you'd expect, and AI drivers would often run into the adamantium walls known as trees.
Dead Rising's survivors were, to put it bluntly, idiots. Half the time, they never followed you or would run off on their own, and giving them a weapon would sometimes result in them attacking you by accident. The sequel improved the AI significantly. Then they slightly switch back to the way survivors were in the first game in the remake of the sequel...
Yayandas: We are about to calibrate the newly installed, super-responsie inertial damper. You will never again feel the slightest shake, and never once be torn from your sleep, even if you are rammed head-on by a Xenon. Nopileos: Rrrr... do they do that? Yayandas: So one hears...
Enemy mobs in Minecraft were quite stupid back in the early days of the game. They were programmed to do nothing more than walk in a straight line to reach the player and jump whenever a block stood in the way. This meant you could lead the infamous creepers to their deaths by tricking them to walk off a cliff or into a lava pool. A patch enhanced the mob AI and they no longer have their quirks.
The tameable wolves update got a lot of flack when it was first introduced for the stupidity of the wolves. They would frequently get lost and de-spawn, or decide the shortest route between two points was a straight line right into a pool of lava.
The AI in GTA IV is usually half-decent, but if you wanna see it spatz out get into a taxi, use a trainer to raise your wanted level and tell the cab driver to hurry up. Watch as he tries desperately (and usually surprisingly successfully) to dodge police cars, only to ram some immovable obstacle and stand there like an idiot while being pelted with gunfire. It usually takes a while for the cabdriver to react and engage reverse, by which time there's typically a massive pileup of police vehicles behind the taxi preventing it from moving. Usually results in a dead cabdriver and a mountain of cars desperately calling for a grenade tossed in their midst.
The criminal groups you are allied with in Just Cause 2 don't seem to understand the concept of "strength in numbers" - faced with a propaganda trailer which will have no fewer than five Panauan soldiers patrolling around it at all times, your allies will gleefully send one guy to take it out. That one guy is usually shredded before he can exit his vehicle.
On the other side, for their amazing skill with firearms, Panauan soldiers cannot shoot you if there is a chain-link fence between you and them.
In Saint's Row 2, pedestrians will often jump to one side if they think the player will drive over them. However, at least as often as not, they throw themselves headlong onto the street, where they're likely to get run over by another NPC driver, or by the player if he was only barely on the sidewalk or if he was only taking a brief detour onto the sidewalk.
There are also certain roads that cabs seem to have... trouble with. More specifically, they become, to borrow Yahtzee's phrase, 'pants-on-head retarded'. The cabs tend to spawn at the end of a long, straight road... then turn around and start driving off in a random direction, taking the longest possible route to get to you. If they don't just explode. Or sometimes they'll spawn, but, for some reason, immediately shift into 'normal' NPC cabs which you can steal, rather than ride in. Also fits as a Good Bad Bug.
Airplane pilots seem to be a panicky lot in SR2, too. Shoot them once (with any gun) on the runway (which is the only place to really find NPC airplanes), and they'll immediately veer off the runway and crash into the closest bit of scenery, usually exploding in a giant fireball.
If you follow one of said planes into the air, you'll see it level off and fly unconcernedly right into a cliff, where it explodes. All planes from the airport explode on this cliff, at the same spot each time.
When you attempt to steal a vehicle that is also on your junkyard list, not only will the cops psychically know and immediately pounce on you, but the driver of said car will invariably panic and veer off the road in a random direction. The chance that this mad dash across pedestrian zones and off cliffs ends with the car despawning, crashing into a semi or disappearing into the ocean is proportional to the length of time you had to wait for it to show up in the first place. Oh, and if he hits something black and white and blue then you get wanted stars. It seems the only way to steal a car on your junkyard list without dealing with this is by shooting the driver.
Get a police car, drive on the rightmost lane and activate the siren. Cars in front of you will turn onto the sidewalk to avoid you. Cars in the middle lane will turn across your lane and stop there. For additional fun, try this on a highway bridge and watch the semis run the civilians off the road in their dash to get out of your way.
The nuclear plant island should never be navigated by car. It is crammed with security vehicles that will do things like accelerate at high speed out of a side street and plow into you, rear-end you when you stop for a red light, or run over a pedestrian and proceed to chase you for manslaughter. It is virtually impossible to spend longer than a minute in this area at the wheel of a truck without getting wanted stars.
Grand Theft Auto: San Andreas features pretty solid AI in most cases, but it breaks down in some areas. On the freeway, the AI can't seem to handle the speed at which it drives, resulting in a lot of accidents, even with no player intervention. If the player stays put long enough, massive pileups and riots inevitably occur and don't end until the player leaves the area.
Civilian drivers are actually dumb cars-on-rails until nudged, shot, or otherwise "awakened", at which point they become truly AI controlled and subject to proper physics (almost certainly for performance). In places, the map's "rails" seem to be set up wrong, and vehicles either accelerate or turn well beyond their actual capabilities, or outright spawn facing the wrong way then tween into place. Freeway pileups are usually a result of "rail" and "true" vehicles interacting badly.
A similar sort of thing seems to happen in areas with particularly steep hills, especially San Fierro. And it. Is. Hilarious.
Also, pretty much every car that needs to make a right turn, is going to do so from the left lane, and vice versa for left turns. And that seems to be the most basic rule for the game's driving AI, but apparently it wasn't. It seems like the only realistic thing the other drivers do in the game is to high-tail it out of there when if they hear gunshots.
If you engage in a gang war, sometimes the enemy gang members will run down to the end of the block just to do a U-turn and run on the other side of the sidewalk. Sometimes this ranges to being miles away from the actual war zone but if the game is savvy enough, you're rewarded with the next wave or getting the area. Most of the time though you're stuck waiting around for them to come back because if you try to leave, the game pressures you to stay there.
Cops who in no way can get to their original car, will usually run out onto the street and jack a civilian's car and drive off in that. Or more hilariously, a fellow officer's car.
Dubbed the suicidal photographer, this fellow stands at the edge of a cliff taking pictures of the city nearby. After he's done he just walks in a straight line into the water and dies. This happens everytime.
Emergency vehicles make no effort to avoid civilians and will usually run a lot of peds over just to save one. Aggrevating when it runs over a mission important NPC.
Unlike the previous games, San Andreas averts Super Drowning Skills, and CJ can swim. However, this isn't extended to anyone else, and if you have a Wanted level, there's no end to the line of cops that will jump in to get you and immediately drown.
It is not just San Andreas, either. Much of the Grand Theft Auto series has apparent Artificial Stupidity, though at the same time, much of it's ambiguous whether it was a matter of programming or of deliberate portrayal. Grand Theft Auto II features fellow carjackers who drive into cars already on the verge of exploding, civilians who run around in circles when a tank is driving through an alley they are in, and cops in a vehicle running over cops who are pursuing you on foot. However, given the nature of the GTA series, one should not rule out the possibility that they are portraying people that way on purpose.
Planes are also tied to 'rails'. This frequently makes them disregard tall buildings, trees, hills or other particuarly tall objects.
NPCs who are falling into the water know how to escape their vehicle and swim, but fail to grasp the concept of finding a staircase or beach to exit the water. They mostly waddle uselessly next to a ledge.
NPCs often crowd around scenes of carnage. This would happen even if the scene involves a flaming vehicle which might go off at any minute.
NPCs on fire never have the ability to stop, drop and roll that the protoganist has. They often just run around until their health runs out and dies.
In Endless Ocean Blue World, one of the ways you make money is by guiding clients around the various areas and showing them the creatures that live there. Clients will always head straight for you, regardless of obstacles. If they cannot do this, they "can't" find you, even if they ought to be able to see you, and you have to go back and find them, and swim in such a way as to keep that from happening. It's very easy to lose someone at the doorways of Valka Castle, or the pillars of the Ice Cavern's Hall of Radiance.
The allied ships AI in Age of Pirates 2: City of Abandoned Ships often interprets 'Sail Away' as 'Sail directly towards the most dangerous enemy ship regardless of wind direction or combat readiness'. Hilarity Ensues.