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.
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.
Generally anyone who sells their soul in Warhammer, 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. If three dwarves manning a cannon are killed by ogres, do they find a small group of ogres and ambush them? No. They hold a massive pitched battle to avenge the grudges, then write down all the casualties of that battle in the Book of Grudges, to avenge next time. How well is this working? They're dying out. Essentially, it's natural selection in action.
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 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.
In Warhammer 40000 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,+
they often fall apart." +
Not true without some major application of firepower.
"Eldar technology is antiquated,+
Eldar craftsmanship is inferior to our own."+
"Beware the tau - they'll sacrifice your babies to their gods!" +
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.
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.
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.
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), wake up the Necrons, and 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.
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 turn you into a human (or appropriate species) vegetable.
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 Battle Tech 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.
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 Incan 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.