You can use asteroid fields to help pare down unfavorable odds, by leading the enemy through them. The AI is, in general, really dumb about avoiding those floating bits of astrogeography, and will gladly suicide on them.
Especially in the earlier games, the player's wingmen were a little too enthusiastic about shooting down the enemy; i.e. : they would cheerfully ignore that the player was in the way... At times, it seemed like when your Wingman was Maniac, he did it on purpose.
The spinoff Privateer 2 had completely incompetent enemy AI. Spoony, in his review, parked his ship next to an enemy and waited to see how long his unmoving, unacting ship would take to be killed. After half an hour in which the pirate managed to wear down the shields once, he gave up entirely.
Sims tend to get stuck at a doorway, unable to decide who goes through first for several minutes. In the original game, this bug used to strand entire groups of Sims at the top of staircases. Sims have been known to starve to death because they couldn't take turns.
Ironically, Sims actually do have good pathfinding when it comes to things less complicated than, urm... doorways. Try building a maze, for example, and the pizza boy will walk right through.
It should be noted, though, that Sims 1 Sims will almost always take the path with less doors to get to their destination, even if the destination is on the other side of a door right next to them.
Sims are, however, known for such suicidal stunts as, when both hungry and tired, waking up to go eat, then passing out from exhaustion, waking up because they're too hungry to sleep, then passing out because they're too exhausted to eat, in a vicious cycle that generally ends in sim ghosts.
However, that's not the AI's fault, but the oversimplified needs system.
Sims will also drop a baby on the floor in front of the refrigerator, then complain that they can't get to the refrigerator to get the baby a bottle...
And let's not forget the Sims who whine about being exhausted, then decide there's no better time for a swim.
Try giving your Sims more than one kitchen (especially on different floors) and watch as they dart between the two in order to prepare a single dish. Alternatively, make the only food preparation area available an outside barbecue, and occasionally the AI will have a sim go for a midnight snack — so, having no other option, they will go have a cookout in the middle of the night.
Sims have sometimes been known to go to sleep, and wake up to yell at the player that they're sleep deprived.
Happily, as of Sims 3, the AI has improved immensely, and Sims are entirely capable of handling the basics of living.
Well, to some extent. Give a home with two adults and a child only one bed, and anyone left without a bed will complain at those using them. There is no check for the appropriateness of doing so, so the husband may find his wife and daughter sleeping in the only double bed, and order the wife out of it. Thankfully, this never goes where it sounds like it’s going.
Oh, and Sims 3 sims will happily sit and eat their dinner on the toilet if there isn't a spare chair. Hey, if you can sit on it, it's fair game, right?
In both the Sims 2 and 3, sims will often, if left to their own devices, leave babies outside on the ground instead of putting them in a crib.
In The Sims 3, the in-game story progression can lead to this. Often, when switching to a family, you may find the entire family at the beach at 10 AM on a Tuesday, despite the fact that all of them should be at work/school.
Whenever an object catches on fire, a Sim's typical reaction will be to run around the object frantically until they catch fire themselves, as opposed to running away from the object or attempting to put it out. Trying to cancel the action and getting the Sim to safety is almost futile, because they will simply override it and resume running around the object. The Sims 4 finally averts this and as soon as a fire breaks out, Sims will automatically run away from it and retreat to a safe distance.
The Sims Medieval lampshades this in its opening cinematic: "People are dumb." Medieval Sims seem to have some of the same stupid traits as their descendants when it comes to pathfinding (they think a Sim is "in the way" when they're trying to get into the Throne Room, even though the Throne Room's doors are twice the size of a Sim), but at least they never start fires when trying to cook.
Sims are pretty dumb on the macro level as well, as anyone who's ever played SimCity 4 can attest. The stupidest is definitely pathfinding, where for various reasons, Sims always take routes in a manner that tends to create absurd traffic jams, particularly at the city limits. The good news is that there is a mod that fixes this. On SimCity (2013), however, the modders were never really able to fix this completely.
The AI in Gemfire sometimes gives up the chance to seize the player base and refrains from attacking with its 5th unit at all. Even worse: they don't seem to be able to grasp the fact that their base is under siege, and they will set a unit on their base and surround it with fences, thus being an easy target for Archers. Not to mention the computers will try to form an alliance with you right before they're about to die at your hands... only to cut the alliance when it's just you and whoever you're allied with.
The animals and staff in Zoo Tycoon can be unbelievably stupid sometimes. For example, animals will be unable to find food when there are three piles of food right next to them, or zookeepers will not be able to get to poo and clean it up for no reason at all.
RollerCoaster Tycoon was particularly bad with this. First of all, unless you destroy the paths at strategic places, every Guest in the game will go wandering off miles beyond any sign of civilization and then complain that they are "lost". Interestingly, they cannot walk ten feet on an unpaved surface, meaning that if you create a path with two dead ends, the Guests will just walk back and forth until the end of time. The Handymen Staff were pretty bad too. Though all you wanted them to do generally was sweep up puke, if you left the command on for them to mow lawns, they will wander off spending hours mowing the endless acres of your theme park while your Guests swim in rivers of their own vomit. And mowing lawns is pointless anyway, because there's a glitch in the game that solves it with ease. If you just touch a piece of land with the landscaping tool, even without actually doing anything, the weeds will disappear. Luckily, the sequel just turned the Mow Lawns command off.
The "No Entry" signs in the expansions solved the problem of guests running off in the first game, but still doesn't keep them from getting lost. Guests will claim to be lost while pacing in front of the exit.
Mechanics also had this problem. If the paths aren't laid correctly, your rides might spend a while not getting fixed because your mechanic spends his time pacing around some other area in the park trying to get to the ride.
Try building a central square with shops all around and watch the visitors aimlessly mill about...
The various boat rides on RCT 2 have a habit of making all the boats cluster around the docking station, blocking each other out of it, with none of them able to enter.
And talking of Chris Sawyer... the AI in Transport Tycoon was legendarily incompetent at the art of building railroads, frequently levelling entire mountain ranges (and suffering no ill effects), and often spiralling stations several times over while trying to get the ends of a line to meet up. Reportedly, this was due to a compromise between processing time and AI lookahead. Having the AI build from both ends at once, effectively meaning both are aiming for an unpredictably moving target, probably didn't help either.
Without going into too much detail, let's just say that in Creatures 2, the Norns that come with the game are morons. Thanks to a problem in their digital genetics, this gets worse after their first real-time hour of life (the so-called "One Hour Stupidity Syndrome", based from the fact that the reward values in the Norn's artificial brain are so screwed up that they're most likely to be eventually sitting around deliriously happy while starving to death in the middle of a forest fire). A player may find that in order to make any progress in the game whatsoever (with getting pickups and exploring and the like), they'll have to micromanage one Norn and spent a distressingly large amount of time luring it into the water so as to pick it up and make it go where the player wants it to. There's a play style called the "Wolfling Run" where you hatch a bunch of Norns and leave them to their own devices — since the default Norns are outsmarted by buttons and fail to connect hunger with the need to eat, this is an exercise in genocide. The modified Norn "Socrates", designed specifically to remember none of its experience of reward or punishment for its actions, essentially ruled by random instinct, ended up prospering much better than its default cousins, granting insight that allowed the game to be fixed.
Game Mods fixed this in C2, but even in the later games, Creatures still tend to gravitate toward "charming" over "clever." Nearly all of the Creatures games feature "wallbonking" — the continued attempts of a Creature to walk through a wall, despite their initial failure to pass through it. They also do things like attempt to eat machinery or ignore food because there's something shiny right beyond it.
Space Colony has the problem that if a character’s shift is over they will ignore the job responsibility, even if that is defending the base from aliens or keeping the air supply running.
For some reason, the titular Pikmin sure do love to drown themselves when you try to cross a bridge with them.
Star Wars: Rogue Squadron for the N64 does this big-time. Your allies are completely, absolutely useless. All they do is fly around, sometimes in circles, leaving the player to do the work of an entire squadron himself. Of course, the enemy is not much better. TIE Bombers especially suffer from this: they always fly in a straight line and never even attempt evasive maneuvers, making it ridiculously easy to shoot them down. On the other hand, the TIE Interceptors in Moff Seerdon's Revenge, who shoot at angles their cannons can't hit any other time, are cheating bastards.
It doesn't get any better in the sequel, Rogue Leader (suggested alternative: Rogue Suggester), wherein any command given to your squad is usually interpreted by them as "Fly very slowly near the turbolasers". They also delight in using lasers against enemies vulnerable only to special player-only weapons, or shooting up the unbreakable walls between them and their target instead of flying around to the other wide-open side.
All of this really wouldn't be quite as bad if not for one notable reason: don't ask us why, but the stats of your AI allies also affect your end-of-stage stats, including whether or not you get a medal. Perhaps as a result of this, it's possible in the second and third games to order your allies to retreat. If you're smart, you'll do this as soon as possible.
In Rogue Leader, you also have Darth Bob the Suicidal TIE Pilot. You could be minding your own business and then suddenly BOOM collision with a TIE fighter out of the blue, and you never saw it coming.
In Rebel Strike — the third game in the series — enemies tend to fly directly head-on into you to attack you; often willingly placing themselves into your crosshairs. This was likely done to compensate after the AI in Rogue Leader damn near had magic aiming powers and could barely be shaken.
The ancient Star Fleet Battles simulator BEGIN 2 has surprisingly good AI. Except for the Romulans. These guys will set off their self-destructs occasionally for no obvious reason, often destroying other ships (friendly and enemy) nearby. As the Romulans can also cloak, this makes fighting them like going for a walk in a minefield.
The AI-controlled ships in Star Trek Klingon Academy suffered from a total lack of spatial awareness. This meant that if you were battling the enemy near an asteroid field, all you had to do was fly into the middle of the field and sit back as the enemy ship(s) plowed into every asteroid nearby and most likely ended up destroying themselves.
The original idea was that you'd be limited in the amount of micromanagement you could do per turn — you basically played as the ruler of an empire with a horrible bureaucracy. That turned out not to be much fun.
The "Auto-Build" option on colonies, while useful, often results in your 1 population, mineral poor colony attempting to build the most expensive building (the Star Fortress), which takes 50+ turns, before industrial buildings that will help other things build faster.
AI ships in space combat also love to uselessly fire energy weapons at planets with impenetrable shields.
AI ship design aims for a Jack-of-All-Trades design that has a little bit of everything for every possible use, resulting in ships that often are unable to press the advantage against a weakened foe before they can recharge their shields in the next turn.
The ants in Bugdom will throw their spears, then run to fetch them. If you can get them to throw a spear through something, they will just sit there, running against a log drinking straw. In later levels, Fake Difficulty comes into play as the ants gain the ability to return from the dead as invulnerable ghosts, still thirsting for your blood, as a way of counteracting this kind of thing.
The propensity of AI-controlled ships in the FreeSpace series to crash into other ships that are in the way has become a running gag among fans. Other idiotic things the AI loves to do include continuing straight on an attack run even though the player is behind them and firing, flying directly into beam cannons, firing beam cannons at enemy capital ships even if friendly units are in the way and will be annihilated, and firing torpedoes from the longest possible range, making them easy to intercept and shoot down before impact.
Also amusing in FreeSpace: if there are no objectives to go for or enemies to attack, all wingmen will return to formation. It's entirely possible for them to get into formation so closely that two wingmen will be constantly crashing into each other; flying in a straight line with their ships touching and damaging one another until their ship explodes. You can't help it either if the AI has decided to flat-out ignore your commands, which they often will when idle.
In Animal Crossing, some villagers are less than bright with trading one item for another.
"Everything is worth what its buyer will pay for it." Besides, your NPC villagers have no concept of a resource or needs system, they're trading with you out of neighborliness.
The Main Street visitors in New Leaf are programmed to say certain lines, depending on their personality type. One of the things a Snooty visitor can say is complaining about how crowded Main Street is. She'll say this even if there's nobody around besides her, or if you find her by herself in a museum room.
Your mech wingmates in Steel Battalion can barely navigate the map, though, when they manage to keep up with you, they can be useful cannon fodder.
In F/A-18 Hornet, your wingman is pretty much useless, and the planes you have to escort aren't much smarter.
This is the biggest complaint people have had about From Dust, where the villagers' pathfinding AI can be a pain in the ass to manage. Most of the time, even the slightest obstacle will cause them to either take a massive detour, or start begging you for help while they stand still in bewilderment. Walking straight into streams of lava doesn't help either.
Aerobiz: The AI would continue to purchase small counts of outdated, inefficient airliners even after newer, cheaper, and more efficient planes are made available. They would regularly place the largest, most inefficient airliners in its fleet on low density routes and then leave them there despite losing big bucks and its competition (you) opening the same route with a small, high efficiency airliner and turning a profit.
Elite: It was less stupidity of the pilot and more stupidity of the space traffic controller/random event engine, but passenger shuttles would periodically launch from space stations regardless of surrounding traffic. Even if that traffic was you, less than a second away from docking (and yes, incoming and outgoing traffic used the same lane). While the collision wouldn't destroy your ship unless it was already damaged, it would destroy the shuttle — and hit you with a massive bounty for criminal activity, to be collected the instant you left the station.
Elite: Dangerous: When it comes to avoiding collisions, the AI tends to be less than stellar, which can generally lead to lulz (or a free kill, since death by ramming doesn't incur a bounty on the killer) in most cases. The patch notes for version 1.1 also noted fixes concerning Asteroid Miners mining nothing. In the HorizonsExpansion Pack, the AI sometimes has trouble with planets, particularly in the larger vessels. It's not uncommon to watch the resident cop take off, pull and immelman turn, and then plow face-first into the ground at mach 1 while trying to attack a rover.
The AI allies in Tom Clancy's H.A.W.X. are incredibly reluctant to actually use their missiles on targets you've sent them after, and they refuse to use guns if they still have access to said missiles, which overall cripples them horribly. About the only time they approach usefulness is in missions where, for plot purposes, everyone on your side is restricted to guns-only, which is the point where they shred everything.
Spacebase DF-9: Workers asphyxiate themselves by not heading back to the airlock in time. More devastatingly, there's the "repair" function of technicians, which, as of Alpha 1b, tends to cause more damage than simply demolishing the damaged item and rebuilding it as a result of the fire caused by the ensuing explosion.
X-Wing had one mission where you were in an A-Wing and charged with immobilizing a frigate. Since your A-Wing doesn't have ion cannons, you had a group of Y-Wings with you. Not only were these Y-Wings piloted by complete schmucks in the area of dogfighting — they weren't even smart enough to realize that their ion cannons depleted the frigate's shields at a slower rate than their blasters. Since the frigate is CONSTRUCTED FOR STARFIGHTER DEFENSE, this mission usually involved telling your "wingmen" to stand off outside 6 klicks while you soloed and 1) Defeated the entire 36 fighter TIE wing the frigate carried, and 2) Reduced the frigate's shields to zero by flying in and blasting like hell and then bolting out of turbolaser range again and again. This was made even stupider (and more frustrating) by the knowledge that the Y-Wings carried about 30 proton torpedoes — if they'd used those, the frigate's shields would have been reduced by 95%. Grrr!
In Airfix Dogfighter, enemy planes very commonly crash into walls. Then again, you'll find yourself doing that every now and then as well.
In TIE Fighter, there's a mission where your TIE Bomber must locate containers with construction materials. The Rebels zip-in and attack both you and the containers. However, the rebels jumping in is triggered by you scanning one specific container, which comes last in the Targeting Computer's list. This can work two ways: either you scan the containers out of order and get ambushed early (leading to swift mission failure), or you wait to scan the container until all of the others have been picked up — so you're free to focus exclusively on the rebels.
The final level of Battle 7 has you rescuing Emperor Palpatine from an enemy escort shuttle, which you must disable. Sometimes — though rarely — your wingmen will attack this shuttle and blow it up, causing you to fail the mission — you have to order them to ignore the shuttle.
If you're doing an escort mission and there are no enemies in the area, allied starfighters will fly in circles. If you ask them to report their status, they'll say that they're patrolling the area.
Playing the Combat Simulator in X-Wing Alliance can be a nightmare. When you destroy an enemy ship, it will immediately respawn... and smack right into you. If you’re flying an unshielded ship, you're good as dead and your score will take a huge drop. Moral of the story, disable respawning.
In all MechWarrior simulation games, this is the only reason the player can win most matches that aren't against much weaker opponents. While you do strategic attacks against specific parts of enemy Mechs to reduce their threat rating in the shortest amount of time and consuming the least heat possible, they'll randomly spray you with often-poorly-chosen weapons at a much slower rate of fire than their heatsinks should allow. This was most obvious in MechWarrior 2, where enemies would spend most of their time walking about while looking at you and only occasionally letting a couple shots loose; fortunately, later games in the series have made it less obvious (though still difficult to ignore).
You can hire field hands to take over a task for you, freeing you to jump in some other vehicle and do something else. The game does not abstract this; you can stand there and watch the worker run a fertilizer sprayer over your field row-by-row, just as you would have done, in real time. But do not ask the hired help to plow for you, especially if you have a rather large plow. Plows, you see, are about the only implement in the game whose "working edge" is not a straight line perpendicular to the tractor's heading. Instead, each blade is set behind and to one side of the last, so the plow works a long diagonal. Used properly, the leading edge needs to overshoot the end of the field by quite a bit before a row is done. The A.I. can't seem to grasp this, so it ends each row early and leaves a saw-tooth pattern along the borders. This is forgivable, considering that if your plow is long, and the edge of the field is cramped, doing a proper job with natural intelligence can be quite a challenge.
A Farming Simulator 17 map is a small town, with many fields and a network of roads between them. Optionally, you can include civilian auto traffic on the roads, and these are a menace. Not only do they ignore stop signs and speed limits (generally choosing a random speed far lower than the limit), they barely react to a ten-ton combine harvester blocking the road. Doesn't matter how far away they can see it, even if your hazard lights and a spinning emergency light are on. If they brake with enough time to avoid a collision, you got lucky. And they will never back up. Even if your vehicle is immovably wedged between opposing lanes of traffic, the civilian's answer is to—no lie—lay on the horn. You may be tempted to solve this problem with a forklift, which absolutely works when your own vehicles need repositioning, but don't bother. These cars have one-way physics immunity, and you'll flip the forklift instead. And tragically, even revving a chainsaw outside the driver's window does nothing. These guys have nerves of steel.
Dwarf Fortress. Strangely enough, part of the game's charm has to do with the fact that your dwarfs are utter idiots. However, they make some choices that lead to... odd happenings. Given that Dwarf Fortress is still in development, many of these stupidities have been or will be fixed.
If an executioner doesn't have a weapon to kill a prisoner with, they don't let that bother them, and kill the prisoner anyway. By biting them to death. (Fixed.)
Similarly, hunters who run out of bolts will gladly bludgeon the animals to death. And if they somehow lose the crossbow, they will gladly (and oddly successfully) wrestle muskoxen and elephants to the ground.
Then, there's sieges. The most common kind, goblin sieges, may leave players overconfident because the average goblin siege charges right into your traps and lets themselves get slaughtered. Eventually, you face human sieges where they, well, do a proper siege. They sit right outside of your crossbow range and wait for your dwarves to run out of food and water and/or tantrum and start slaughtering each other. Any attempt to foray is met with a withering storm of bolts and arrows which, because DF averts Annoying Arrows, is very deadly indeed.
An odd combination of stupidities leads to hilarious results: "Ignis promptly starts to spar and get a punctured lung. Instead of being a good wounded dwarf and staying in bed, he promptly walks around the fortress falling unconscious, refusing any medical care whatsoever. This I could tolerate because it meant the idiot would be dead soon and no one would care. After traveling to my royal dining hall in just under a year, he proceeds to grab a plump helmet stew from my nearby food stockpile. He then promptly falls unconscious again and drops his food in the hallway. The stew proceeds to rot and create a gigantic amount of miasma and there is nothing my dwarves can do about it since the stew is owned by Ignis. After waking up half a season later, Ignis, seeing his stew has rotted, proceeds to the stockpile once again to grab some cat biscuits. You can see where I am going with this."
When there's a siege or other such hazard present on the surface, you can order your dwarves to "stay underground" to keep them safe. The way the dwarf AI does this is to continually check "am I aboveground?" and if so, cancel whatever task they were doing. The jobless dwarf will then pick a new task from the list of available tasks... which is often the very task they just cancelled. The result is known as the "entrance dance", where a huge crowd of dwarfs winds up clustered around the entrance constantly jumping back and forth through it and announcing cancelled jobs. Most players will have to design their fortress with an outdoor courtyard of some sort to keep these idiots safe. (Fixed with the replacement of the "Stay Underground" order with the burrows system.)
A variation is when you have a dwarf or two outside doing a job when an enemy of some sort shows up near the door, but without being able to reach your dwarves, i.e. because you have a walled-in courtyard. Dwarves are dumb enough to run away from any enemy they can see, even if they can't reach them, so they will cancel their job and run away, most likely into the corner of your keep rather than back inside. The job cancellation causes another dwarf to pick up the job, which will take them outside, where they see something scary and run away, cancelling their job in the process. Repeat until your entire fortress is panicking in the far corner of your completely protected, walled-in outdoor keep.
Siege engines such as ballistae are operated by civilian crews, not military dwarves. That means that if you order your civilians to hide inside while your military fends off a siege, and the ballistae are outdoors, they'll abandon their posts. They'll also abandon their posts and flee if they see a hostile enemy. Doesn't matter if the enemy is across a moat and through an impenetrable fortified wall, and that they're currently manning a contraption that can slaughter them all in a single shot...
When dwarfs are digging trenches or building walls, they have a universal preference for which side of the wall or trench they stand on while doing their work. For example, they prefer standing on the west side of the tile if that space is available. So one must be careful about how you set up your construction orders, or the dwarfs can wall themselves up and eventually die of starvation or thirst. (Fixed, at least for the most part.)
Nobles have an unfortunate tendency to mandate the production of items your dwarves have no hope of producing, e.g. windows in a location without sand. Their stupidity frequently leads to beatings and imprisonment for skilled dwarves who lack the raw materials to work with.
In previous versions, a (now resolved) bug could result in mayors ordering themselves beaten for failing to satisfy their own mandates.
In an example of If You Die, I Call Your Stuff taken to the extreme, the infamous BoatmurderedLet's Play features dwarves rushing to loot the possessions of their fallen comrades in the middle of an elephant invasion and getting trampled to death, only for additional dwarves to rush for their loot...(Arguably fixed, as players can set objects of dead dwarves to be automatically forbidden when they die.)
Dwarves have been known to steal the clothes from a comrade who burned to death. While said clothes are still on fire. I lost a whole fortress to that one. (Again, arguably fixed.)
From an older version's release notes: "stopped people from giving quests to kill themselves." (Obviously, fixed.)
Prior to the Adventure Mode overhaul, you could receive quests to slaughter demons who had risen from the depths of the Earth and taken control of a civilization. From your quest-giver's civilization, usually. Doing the deed would make said civilization your enemy, including the one who had given you the quest in the first place. One better, the involvement of your companions or the demon attacking other peasants may well start a "loyalty cascade" where everyone starts killing everyone for killing anyone.
Since the ability to target specific body parts was implemented, there have been many cases of units fruitlessly punching, kicking, or biting their enemy instead of using their weapons because they randomly get some moves (involving a random kind of attack and body part) very high chances of connecting without factoring in if it could even do any damage.
Similarly, wrestlers have a habit of repeatedly grappling a body part, then letting go to grab another part, instead of following up those grapples with any actually damaging moves. Particularly annoying when you know how devastating a wrestler who actually knows what they're doing can be in Adventurer mode.
A dwarf got a small cut on his arm, which needed to be cleaned and bandaged by a medical dwarf. A second dwarf with no medical experience was drafted to take care of it. The newly minted doctor looked at the small cut, diagnosed the patient as having rotten lungs, and performed surgery to remove both lungs. The patient did not survive the operation. (Source)
It counts as a subversion as it is an implemented, deliberate stupidity. However, dwarves — even with no medical experience — should be able to tell that A) their lungs were not in immediate medical need, B) they just had a cut that need patching, and C) lungs are vital for patient survival. That still goes too far, even for inexperience.
Dwarf tries to "attend meeting" with siege operator, stands on ballista set to "fire at will," gets injured.
Goblin leaders have been known to turn up riding on tamed giant toads, which is mildly unnerving... until the toad hops into a murky pool and the goblin drowns.
When crossing a river, the AI only considers how deep the water is, and ignores how fast the water is flowing. Due to a quirk in how liquid flow is simulated, water is only leg or ankle deep at the lip of a waterfall. Thus, everyone tries crossing there, and gets swept over by the current. ("Fixed"; Natural waterfalls no longer have ramps in places that allow dwarves to traverse the lip.)
Mothers carry their babies with them everywhere... even into battle.
If said mother loses their weapon, they are prone to begin hitting the enemy with their own baby.
If an invading army spots one of your pet cats wandering around outside, they will make sure that cat is killed, even if doing so requires that they ignore your fortress as the entire army spends days on end chasing the single cat around. Usually, this only happens if no other targets are available, but it's still questionable prioritization.
It's very possible for a brewer to completely ignore the fact that the entire fort is dehydrating to death because the caravan that you bought all the cloth and leather off just left. It's also possible for said brewer to stop brewing to throw a tantrum because he's thirsty.
Vampires will try and frame another dwarf for the death of someone they just drained dry. Problem is, their parameters for picking someone to take the rap were originally a little odd, ending with them accusing babies or even livestock of murder. That particular bug has been fixed, but they still aren't very good at working out the plausibility of their accusations, sometimes leading to them accusing a dwarf that, by the game mechanics, cannot possibly be a vampire, such as one of the starting seven.
Actually, it is possible for the starting seven to become a vampire from drinking water contaminated with vampire blood, so this isn't quite as impossible as it might seem.
If the pack animal a merchant is leading falls unconscious, the merchant will drag it along the ground instead of stopping. And if the animal was rendered unconscious by a bandit, the bandit will keep beating the animal as it's being dragged, instead of attacking the merchant.
While dwarves used to take to the hospital until you died of old age, now they favour a model where they get up and start working as soon as everything is sutured and splinted, even if they're suffering from acute blood loss and will die within days.
Dwarves in taverns seem to have a problem with realizing they have booze in their cups. They've been known to drink from the barrel with a perfectly drinkable cup full of wine in their hand, then put the cup back in its container, dumping the booze in the process. Or occasionally just carry it everywhere, repeat the process with two cups, and end up with their hands full.
A related bug causes them to never consider drinking directly from the barrel when there's mugs around. Not too troublesome until they lose their ability to grasp, leave for the booze stockpile with a mug they cannot pick up, and just stare at the booze until they die of thirst because they couldn't think of anything else.
With the emotion overhaul, dwarves can feel vengeful about their friends, or fellow citizens in general, being attacked or killed. Unfortunately, this applies to monstrosities like werebeasts and titans, which means anything that killed your citizens close to a burrow will get mobbed by Leeroy Jenkins' extended family, turning a tragedy into a genocide.
With Bonsai Forest's aversion and addition of enormous, climbable trees, creatures have acquired a tendency to reenact the Cat Up a Tree scenario with annoying frequency, climbing into the tree without a single problem then being unable to come back down until they starve. Chopping down the tree usually works, at least, though with risk of someone getting bonked in the head by a log. This can go from just your livestock to your dwarves, being especially stupid in the latter case.
An old one related to the above is the fact pathfinding for flying creatures is utterly borked. Creatures regularly forget about the fact they can fly if there's no ground path to their new location, and will be just as stuck as any other if left with no pathfinding options. Until you actually make the ground path, at which point they fly the actual path they should've taken before like nothing was wrong.
Building destroyers will always seek out buildings to topple above all else. It doesn't matter if there's twenty steel-clad dwarves chopping away at their limbs. It doesn't matter if there's a giant gauntlet of foes they could be taking swings at. It doesn't matter if the building in question is constructed entirely of indestructible artifacts. It doesn't matter if they're undercover and should be acting friendly and not commit any hostilities. It doesn't matter if they're actually friendly and the structures they want to demolish belong to an ally or even their own civilization. They will not fight the compulsion to knock everything down. The Building Destroyer tags should therefore be handled carefully, because intelligent creatures that can join forts and have them will bring no end of chaos until you tire out and murder them.
One so insidious it wasn't really noticed as stupidity until after it was fixed: The coup-de-graces. Pretty much everything and everyone after the combat overhaul aimed at the head whenever their enemies passed out completely. It took several versions and more than one year for them to realize that, just maybe, they should take off their enemy's ☼Steel Helm☼ or other such protection before trying to club them to death, and thus tended to tire themselves out trying to kill the enemy through it, wasting entire days in particularly bad cases.
Marksdwarves have been known to disregard their ammo or the fact their weapon is made to shoot rather than bludgeon to charge into melee range with threats and get slaughtered in order to kill something. Stationing them instead is a must, and even then they've been known to leap over fortifications and off their shooting towers, no matter how high up they are in order to melee enemies rather than shoot them, forcing you to install floors above fortifications so they won't do suicide leaps to charge into suicidal frays. This combined with the ammo use bugs makes them almost not worth it.
Overall, in the same way Team Fortress 2 is sometimes described as a "hat simulator with gameplay on the side", Dwarf Fortress can probably be reasonably described as a "stupidity simulator with gameplay on the side".