Clan Ice Helion historically had a bad habit of rushing into situations without thinking. This resulted in them being one of the weakest of the Clans and they were derided by their fellows as "Clan Tantrum". However they only truly hit this trope when they tried to invade the Inner Sphere on their own; something that seven vastly superior Clans had (at best) limited success with. They were promptly ripped apart before they even reached the Inner Sphere as they ran straight into the Occupation Zone of the much stronger Jade Falcon and Hell's Horses Clans. They now act as a glorified pirate band to acquire resources.
Dungeons & Dragons: The Hand of Vecna is an Artifact of Doom that requires its user to cut off his or her own hand/eye (there's also an eye of Vecna) and graft Vecna's in its place. The artifact grants its user magical power, but has a mind of its own and wants the user to follow its agenda. The hand can kill those who disobey. And those who obey, too! So, you're pretty much screwed either way.
Also, as the Knights remind us, there can be some who don't remember that the Hand of Vecna is a LEFT hand when it's time to do the lopping.
There's also the decidedly non-canonical Head of Vecna, which induced Too Dumb to Live on an entire party in a Crowning Moment of Funny. (Of course, maybe it's just as well this thing was just a joke. Given what Vecna's Hand and Eye have been known to do to folks who use them, just imagine what his brain could do.)
But let's be fair: the Hand and Eye could just as easily be grafted onto somebody who already just happens to be missing a left hand and an eye. Provided they don't know the full and terrible history of the items, they might be classed as tragically stupid instead of hilariously stupid.
Racial Holy War. The "holy white warriors" you roleplay are, put bluntly, so dumb they can be wielding an assault rifle and still be chased off by a horde of Jewish babies and grandmothers (and not Badass Grandmas either).
They'll also accept bribes not to fight in the middle of combat. It gets to the point that people sometimes wonder if it isn't actually a Stealth Parody.
In Rifts there are aliens called the Arkhon, a race of (not proud) warriors. One particular clan has lost favor, and decides to redeem themselves by invading Earth. The reasoning behind this is that the Arkhons had attempted an invasion on Earth a few thousand years ago, and had gotten utterly routed by the magic of the Nazcan Empire. Basically, the plan was to go to a planet that had proven itself to be more than a match for them when said race thought bronze was the height of technology, and has presumably been advancing steadily ever since then, and try and take it over. Even ignoring the fact that the Arkhons didn't know about the insanity that had happening on Earth over the last few hundred years, the Arkhons were more or less committing very elaborate suicide. You'd think they would have stopped and reassessed their plans when they lost a third of their fleet en route, and then lost another quarter of their ships when the orbital community attacked them.
Generally anyone who sells their soul: a boost, albeit a very large one, in one's abilities does not justify becoming a monster. Also note the parents who give their mutant children to Beastmen in Warhammer Fantasy, yes it is understandable as the child would be killed otherwise, but it is stupid because these children have a habit of coming back with beastmen during a raid, then killing and feasting upon their own parents.
And the dwarves. Oh boy, the dwarves. They take Revenge Before Reason to the point where it's firmly in Too Dumb to Live territory. They're all fanatically bound to a code of personal honour that holds vengeance as a sacrament; no matter how trivial the slight or minor the offence, they never forgive and never forget. They actually have a big book where they hold all the grudges they have against the other races, and try to resolve these grudges with big battles. Win or lose, they take the casualties of those battles and put them down in the Book of Grudges as well, to avenge later. As a consequence, they are pretty much constantly in a state of war with everyone and are rapidly dying out. Natural selection at work.
Declaring a constant war on the most powerful mages in the world, and the most skilled warriors (Elves, even if they are dying out), the most numerous enemies (Orcs), the most high tech (Skaven), the toughest soldiers (Chaos) probably do not help the dwarves chance of lasting longer than a few years.
The Dwarven Slayers (Troll Slayers and Giant Slayers) take this trope to Up to Eleven. Justified since they're Death Seekers.
Becoming a submarine sailor is an alternative career for a Dwarf who would otherwise become a Slayer. Dwarves have a notorious fear of seas.
The absolutely mad vampire Konrad von Carstein, although it is probably more of an example of being Too Insane to Live. This is the kind of guy who convicts his own mother for the crime of giving birth to him without his own permission.
Bretonnian Knights Errant. Those Hot Blood young knights are impetuous - i.e. they will spontaneously attack any enemy at reach unless they pass the Leadership check, and they are Immune to Psychology on the turn they charge. The usual result is a Suicide Attack. This tendency acts also as a kind of a Darwinist natural selection - those Knights Errants who survive, are to become the new Knights of the Realm.
Snotlings are not only stupid enough to make Orcs look like warrior scholars, they also tend to imitate the behaviour of other units nearby. If they for example see veteran Black Orcs charging a giant monster, they will happily do so as well, despite their small size and feeble combat skills. No wonder other greenskins think of them primarily as emergency rations.
The Elves got their moments too. Wanna know why the War of the Beard is called that? Meet Caledor II the Warrior, the Phoenix King at the time of that war. Long story short, Dark Elves disguised as High Elves attacked a Dwarf caravan. Gotrek, the Dwarven High King, demanded explanation and recompense. Caledor replied that he would only answer to pleas, not to demands, and sent the Dwarf emissaries away with nothing. This would propably be enough for the Dwarves to start a war, but Gotrek sent the emisaries again. This time the emisaries refused to return with nothing saying they'd rather have their beards shaved off (a GRAND insult for a dwarf). Caledor did just that. And so began a war that ended with both races on the verge of annihilation, Caledor dead, and the prized Phoenix Crown in the hands of the dwarves. There are differences in this story between elves and dwarves but both agree that Caledor, rather than being called Caledor the Warrior, should be called Caledor the Idiot.
Being Too Dumb to Live is epidemic almost to the point of an actual infectious disease. The Imperium is the most common offender (having among other things ignorance and blind fanaticism to government policy might be a reason for that) but by far the worst are any and all servants of Chaos, a faction that performs regular human sacrifices for anything more complicated than boiling water and is engaged in a perpetual Enemy Civil War due to being, well, Chaos, yet they still have a constant stream of followers willing to embrace madness and slaughter for the miniscule probability of achieving the "honour" of daemonic possession.
While we're on the subject of the Imperium, the Imperial Infantryman's Uplifting Primer. Whoever wrote it seems to have been a lobotomized imbecile with no military, tactical, or mechanical experience whatsoever, assigning hymns to everything such as loading a lasgun or autopistol and throwing a grenade which we all know is bullshit (and the Litany of Stealth, to be recited as you approach the enemy), encourages Leeroy Jenkins tactics, tells soldiers they will die horribly, gives disturbingly awful background information on enemies, giving gems such as: "Ork weapons are extremely crude and prone to misfires or jamming." note Maybe true for the shootas and any other ranged weapons, but as the HERO OF THE IMPERIUM points out, they're effective as the average bolter. And I did see an axe misfire once, but it really wasn't all that likely. Impossible under any circumstances, really. "Tyranid weapons are formed of living tissues,note True. they often fall apart." note Not true without some major application of firepower. "Eldar technology is antiquated,note Technically, true. Eldar craftsmanship is inferior to our own."note Haha, no. "Beware the tau - they'll sacrifice your babies to their gods!"note Nyet—Tau are Flat Earth Atheists to the point of Scary Dogmatic Aliens. It also says Tau are frightened by fire. And water. And thunder. And unnerved by hairy people note Were that true, the Imperium could just send in the Space Wolves and maybe some Valhallans.They are also derived from bovines and chew cud and have udders. They have terrible eyesight so that their hearing overcompensates, allowing you to scare them off with loud shouting. And those guns they're carrying require sustained streams to injure a healthy, armored humannote bullshit, bullshit, and groxshit. First one is racist, second-Well, that's only true if it's with sonic weapons. But that's true for everyone. 3rd-lethal groxshit-Tau guns are some of the most powerful in the game, being very capable of knocking over a Space Marine. Then, somewhere else in there, it says that Imperial soldiers have the best armor and weapons, which will oneshot anything and protect from anythingnote So a glorified t-shirt and flashlight is a better choice than power armor, a chainsaw and bolter? Ah had no idea.... What's dumber than that book? A guardsman that takes it seriously.
Then again, we're still waiting for a Canon Guardsman who treats the "Primer" as something other than a glorified toilet roll... (Ciaphas Cain makes a reference to troopers reading their primers for inspiration or amusement).
Plus, in terms of what's available to most humans the guardsmen's equipment really is very good. A few of the "misinformation" points are also jokes about the setting's Story And Gameplay Segregation; for instance, the bizarre claim that for all their apparent muscle Orks are no stronger than a human actually holds up on the tabletop rules.
Subverted with Ogryns which are are the largest and most physically powerful type of abhuman used by the Imperium. They are so stupid that one got a large chunk blasted out of its chest during a battle and only died when it was pointed out to him later. Apparently this is a common occurrence
Then you have Orks, who suffer the opposite—Too Dumb To Die. They are so dumb that their technology is actually the result of their belief in ignorant ideas actually creates a Psyker Field around them, making reality bend to their will. That's right, their "tek" works because Clap Your Hands If You Believe. Give an Ork a metal tube attached to a box full of scrap and convince him that it's a gun, and he'll be able to shoot with it. The more Orks you convince, the more powerful their technology can become.
Although the Orks are more than capable of falling straight into this trope when they want to, especially when they are guarding things - the go-to response of Ork sentries is usually "charge at enemy firing weapon", rather than, say "raise alarm". Gunfire and noise are generally unremarkable in an Ork camp, so the Orks are usually rather easy to sneak up on, especially if their enemies are competent. It is also common for Ork armies to fall apart because a campfire brawl escalates into a massive civil war in the space of fifteen minutes, which usually ends with the Orks' enemies sitting back, watching the fireworks, then carpet bombing the survivors once the Orks' AA gunners are dead.
Also, the Eldar, who despite their ability to predict the future, managed to orgy into existence the fourth god of Chaos (Slaanesh, god of Excess, in case you wondered), which drove their species to the brink of extinction, wiped out their galaxy-spanning empire and dooming their entire race to having their souls eaten upon death.
There are a lot of planetary governors who hire the Dark Eldar as mercenaries. In recent lore, a Tau hunter cadre also made the mistake of allying with them. Big mistake, EVEN BIGGER regret.
There was an incident where a Tau world was under attack by a Tyranid splinter fleet. Necron warships appeared and proceeded to slaughter every living thing before descending to the planet. The overjoyed Tau sent a big delegation including their Ethereal to meet the Necrons, who proceeded to slaughter every living thing before leaving.
Funny enough it was a few days into the celebration.
On the other hand, the Imperium takes this too far at times. The Tau themselves have made several mutually profitable alliances, so it's obvious that such endeavors aren't always suicide.
Quite possibly the worst offenders are the Adeptus Mechanicus. So obsessed are they by their pursuit of mechanical artifacts that they will happily enter Necron tombs to study the Necrons (who they consider holy guardians, and their Omnissiah is all-but-stated to be the Necron Void Dragon). Ciaphas Cain, HERO OF THE IMPERIUM has had to literally force Adeptus Mechanicus personnel not to touch Necron tombs at gunpoint, while telling Mechanicus tech-priests how horrible the Necrons are, and yet somehow, that didn't work. In the extremely likely event that a Reasonable Authority Figure and Only Sane Man isn't around to threaten tech-priests not to wake up the Necrons, the tech-priests then have maybe a nanosecond of being surprised before being desintegrated. The Necrons now awake the rest of the tomb and start purging the galaxy of life. Thanks, guys.This is not the first time this has happened.
In a setting where ruthlessness is a virtue and mercy to an enemy is often the height of stupidity, the Old Ones may prove to be the biggest bunch of morons around. They had defeated the entire species of both the Necrontyr and the C'tann and could have ended them permanently. However instead of doing the smart thing in 40k, such as dumping them body and soul into the Warp, nuking their planets out of existence, or enslaving them with their godlike mind powers, what do the Old Ones do. They leave both enemies alone. This allowed the Necrontyr to eventually contact the C'tann and supercharge them with the sacrifice of their life-force while the Necrontyr have their consciousnesses placed in near-indestructible robotic bodies. Neither of them make the mistake of taking mercy on Old Ones and wipe them out by constant bombardment of supernovas and black holes.
Having your intelligence drop to 0 in certain RPGs will render you literally Too Dumb to Live and lead to immediate character death. Presumably your character forgets how to breathe. Others just put you in a persistent vegetative state.