Vulture and Greyback, two griffin soldiers in Ace Combat: The Equestrian War. The former is outmaneuvered by Firebolt and killed upon crashing onto a column during the battle of Cloudsdale. The latter is fried by The Burning Talon's laser because he is too focused on trying to kill Lightning Bolt. When he finally notices the danger, it's too late for him.
Dave in Dave Stdider Pokemon Traner. Jake England the evil poacher wheels a ray gun specifically called the "Dave Kill Lazer" past Dave & co. while laughing maniacally. Dave nonchalantly acknowledges this fact and then just ignores the fact that an evil villain has a laser SPECIFICALLY DESIGNED TO KILL HIM. Also he's fooled by Jack and Karkat's false mustaches in Chapter 8, despite the fact that a nameless character (The Ticket Taker Man) notices like, a sentence before.
In Kyon: Big Damn Hero, Haruhi thinks this of Taniguchi after learning about his ratings. Kyon calling him a 'living perv-wiki' probably didn't help.
Dr. Emil Hamilton in Terminal Justice. While everyone else realizes how bad an idea it is to antagonize the embodiment of Death, Hamilton decides to mind control Power Girl into slapping some magical shackles on him. Not only is he already aware of the very real possibility that this would merely prevent people from dying without affecting Death himself, he also knows that Death has a lot of friends in the Justice League, including his girlfriend Supergirl. The end result? Everyone and everything in the universe stops dying (no matter how horrifically injured) while Death is in a coma. Meanwhile his siblings cause widespread destruction (among other things, tornadoes started appearing literally everywhere in the world). Unlike many cases of this trope, Hamilton doesn't actually die. Instead he's so horrifically injured his doctors insist he won't be eating solid food for at least a decade; he's left infected with the plague (courtesy of Pestilence) and with all his backers destroyed (War's doing).
Senator Kinsey in Ship of the Line: An Unlikely Hope tries to threaten Darth Vader (really Xander turned into Vader) while on-board the Executor, completely ignoring that not only is Vader a two meter cyborg with a laser sword and magic powers, but is also in command of a ship that (according to Star Wars fans in Stargate Command) could eliminate roughly 90% of humanity in 24 hours with an orbital bombardment.
In Christian Grey Vs Pepper Potts, this trope pretty much definesChristian. One prime example is when he decides to romantically pursue Jennifer Walters, knowing full well that she can turn into She-Hulk. When Jennifer is creeped out by him taking her to a hotel room after picking her up while drunk and runs away, he has her assistant kidnapped. He then expects Jennifer to have a nice dinner in a warehouse, with the threat of the assistant's knees being broken with a sledgehammer if she refuses. By the time She-Hulk is done with him, his spine is broken and it's expected that he'll never move again. (He does recover, but only with the help of a vengeful Asguardian goddess).
First when Older Shinji offers the chance for them to stand aside despite orders so he can heal Councilor Tevos, they insist that he's only one person and no matter how advanced his technology, he can't take on over two dozen soldiers by himself. Afterwards, others insist they can't just let Shinji do what he wants after he's killed over two dozen of their soldiers.
Later the Bridge Bunnies think they can take on Aria T'loak despite her being dressed exactly like Shinji (or more accurately, he dresses exactly like her), whose clothing is known to stop rounds from assault rifles, implying that she's at least as good as he is. Not to mention the Bridge Bunnies weren't nearly as well armed (having only their pistols) and only outnumbered her three to one.
The Wizarding World is this when Harry temporarily returns to Earth in Wizard Runemaster. In order to punish the goblins for not handing over Harry's wealth, the Ministry of Magic is having radioactive waste poured into their cave systems. Said goblin caves run under most of London and a good deal of Great Britain.
In The Watchman Warren kills Tara, rapes Willow, and tries to kill Buffy despite knowing that Xander has become The Sentry, whose described as "Superman on steroids with more mental problems than Joker". Warren thinks that because his magic orbs let him fight a Slayer evenly he's a match for Xander who treated a Hellgod as an annoyance. Xander throws Warren to Greenland, beats the hell out of him, rips off his head, and vaporizes his body. He then drops Warrens head off at the local demon bar as a warning to anyone trying to mess with his friends.
In the Lost ForeverFuture of Despair comic, Panne fits in through and through. Because "having to give birth to your and Henry's child in isolation per (fanon) Taguel costumes" should not equal to "stupidly running away from a mostly safe CASTLE to the nearby WOODS all alone and thus getting yourself skewered with arrows by a bunch of dumb hunters who didn't know better".What an Idiot!
It's even worse when you recall the now also Lost Forever notes on said "fanon" costumes. These instincts that lead Taguel moms to seek for "complete isolation" when giving birth are supposed to be completely irresistible and unrestrainable, so they imply thatevery Taguel mother is pretty much WIRED to be Too Dumb to Live - not only they could potentially run into hunters like in Panne's case, but they deny themselves any support in case of pre/current/post-birth complications for herself and her baby/babies.DoubleWhatan Idiot!
Flint in Enveloped in the Darkness nearly kills Draco, because he's Harry's best friend and Harry pissed him off. Not only does he brag about it in front of the entire Slytherin house and Narcissa, he then insults her when she gives Harry the right to enact the Malfoy family's vengeance. It apparently never occurs to him that even if Harry wasn't able to easily defeat him, he's attempted to kill the sole heir of one extremely rich and powerful family who pretty much control the Minister of Magic. Due to the rules of the Dueling Pit, Harry doesn't kill him; he just makes Flint wish he had.
A Brief History of Equestria: Roughly half of the Unicorn nobility stayed behind when the Manehattan river valley was evacuated for the new homeland because they couldn't stand the thought of abandoning their hoards of treasure. They were all naturally frozen to death by the Windigos.
Star Saber's little group of thirty who tried to counter-revolt against Hurricane failed because they tried to steal weapons from an empty armory... which was positioned next to an active mess hall... in the middle of the day. Hurricane found this hilarious.
The extremely misandrist stallion-hating Commander Sullamander tried to use her position to pass laws that would effectively outlaw heterosexual intercourse among the pegasi. Wind Whistler points out that this would effectively end their people, and as such attempting to go through with this plan would count as high treason according to the Articles of Command, resulting in Sullamander's immediate removal from office and execution. She very, very reluctantly backs down, much to Wind Whistler's disappointment.
Krysta from My Little Unicorn, who decides to follow a strange orb of light with the voice of the bad guy, promising her to tell her where she comes from.
Nightmare Sky, a minor antagonist in Nightmares, steals the Elements of Harmony and tries to unleash their power on the Mane 6. To repeat, a Nightmare, an incarnation of rage, hatred and disharmony, tries to wield a set of weapons designed specifically to fight disharmony, against the highly harmonious group chosen by said weapons to bear them. Given that Sky, unlike Nightmare Moon, did not have the advantage of having been an immortal god-pony before her transformation, the resultant backfire turned her into a sooty smear.
When the Kurama clan seeks to defect to Iwa in Black Flames Dance in the Wind: Rise of Naruto, one of the chuunin tasked with killing Yakumo's guards figures the medic is dead for sure because he suffered a wound that would bleed out in several minutes. The other clan members are honestly amazed at his stupidity for believing a medic wouldn't be able to heal a wound like that when left alone and list off several different ways to do exactly that.
The entire Kurama clan is this due to the clan head's policy of Revenge Before Reason. They want to defect to Iwa but know the only way it will work is if they have as much of a lead over any hunter-nin as possible. Rather than quietly leave during the one time almost half of all forces are away from the village, he insists they first kill Yakumo who is under 24-hour guard. A guard manages to escape and Naruto's team (plus Anko) are sent in to capture and/or kill everyone.
Tenten in The Darkest Light insults Naruto/Naruichi and Takada within earshot of both. Naruto beats her unconscious (which is the only reason Takada's bodyguard doesn't kill her) then when she wakes up, she's ripped a new one by her clan head before Naruto fights her sword with a wooden stick, spending an hour beating Tenten then healing her back up so he can start over.
In one Naruto fanfic, the Kyuubi released a powerful pheromone when it was sealed that basically made everyone into sex maniacs the moment they hit puberty (extreme horniness is a perfectly acceptable reason to be excused from class and it's considered extremely rude to reject someone if they want to have sex). When he first uses his Sexy no Jutsu, he's flat out told that turning into a naked woman, especially in public, will get him raped. When he says he'd just change back and laugh, he's warned that his attackers probably won't care what gender he is at that point. Naruto promptly vows to never use it again, unless it's really funny.
A couple chapters (and several months) later, several Daimyo guards chase said Daimyo's prey (hiswife) through the tall grass of a preserve called The Savanna. Even if Naruto wasn't actively hunting them, experienced hunters like them should know better than to enter tall grass which has zero visibility on anything less than three or four feet tall. Especially since many of the animals from the Forest Of Death (such as giant tigers) are imported from The Savanna.
Plasma's Folly: Even if the formula hadn't been faulty, common sense says it still wouldn't have been a very good idea for Colby to inject himself with a random syringe he found sitting in a paper bag.
Arnold of Soulless Shell falls for Leif's not very clever bluff when threatened with execution for rape, inadvertently confessing to his crimes while begging for mercy as Leif sentences him to death. It's suggested that had he not done that, Leif might not have had enough evidence to actually sentence him to death. He then breaks out of prison, and not only is dumb enough to want to kill Leif in revenge, but tries to rape a young girl first, pushing Leif's Berserk Button and earning him a messy death at the hands of his One-Winged Angel form. Apparently, he didn't even think to bring the guards he recruited with him, although given how easily Leif dispatches them soon afterward, they wouldn't have done much good.
Paternal: A Spy (separate from the RED and BLU Spies) named Bane is on a mission to capture the BLU Scout on the Administrator's orders. His modus operandi is to corner Scout while he is alone. The past several times, he has only failed because someone else showed up to save Scout. So what does Scout do in Chapter 26? He goes out for a jog in the training room. Alone. At night. Without a weapon.
Later in the Gensokyo 20XX Series, this is played with an age-regressed Reimu. On the account that she cannot sense danger and seems to have an inability to fear, the other characters tend to see her as this, as she is known to place herself right into danger and engage in something reckless. This is further exacerbated in that if told not to and the reasons as to why, she would do it anyway, especially in situations in which she should probably know better or otherwise have some common sense (like "don't put things in your mouth to see what it is", for example).
This is Played for Laughs, in Baka's case, who is apparently stupid enough to fall for Reimu's tricks, the which involves him being tricked into sticking something in electric outlet. From what is said about that, he's fallen for it more than once and never seems to catch onto the fact that is why Reimu tricks him into doing that in the first place.
Ren: "You are stupid as hell if you can fall for a child's mischief more than once."