Name something "Forbidden X" or mark something off-limits in any RP session, and schmucks will flock to it like Santa Claus was inside handing out presents.
Words of wisdom: If your DM stops something you're about to try with "Are yousure you wanna do that?" or something of the sort, it's best to drop it. Death and/or catastrophic derailment of plot and setting may occur. If they ask more than once? Stop, or kiss your campaign goodbye.
On a related note, a pretty standard ploy for illusion-casting villains is to deploy several illusory threats in a row against the heroes, right up until the players are having them attempt to disbelieve everything they encounter. Depending on the game system, an "attempt to disbelieve" might just forfeit your saving throw if a real danger shows up, so once the players are declaring that their characters don't trust what they see, that's when the villain sics a for-real monster or attack-spell on them.
Nearly every encounter in Gary Gygax's adventure Tomb of Horrors fits the trope. In fact, the entire tomb is schmuck bait, using the lure of treasure to provide an evil demilich with the souls of the most powerful adventurers. There's never a shortage of players who want to put their characters through the most infamous dungeon in history, but not only are their odds of beating it incredibly slim, the Tomb won't yield much experience or treasure to characters fortunate enough to survive.
Keep on the Shadowfell has a door marked "Do Not Enter. SERIOUSLY." That's where the slime is. The slime that can kill a party by itself.
Deck of Many Things? Sure, there are some extremely good magic cards in there, but how many gamers can resist drawing again? They will almost always end up regretting it.
The Splat book Tyrants of the Nine Hells mentions a location in Dis, the second layer of Hell, called the Garden of Delights, that appears to be a paradise to mortals who enter; it's a beautiful garden where lovely nymphs welcome visitors and lavish affection, along with food and drink of the finest quality on visitors — all for free, no less. The fact that this place is in Hell should tip people off that it's a trap. Dispater, the ruler of the layer, employs efreet sorcerers to maintain the place, and the "nymphs" are erinyes. The purpose of the Garden is to enspell its visitors with its intoxicating nature to prevent them from wanting to leave, and eventually tempt them to evil. (If the erinyes can't do that because a mortal is incorruptible, they just let the visitor starve; the food is an illusion, and visitors will eventually die of thirst or starvation trying to live on it.)
There's another one, down in the fifth layer, Stygia. The Pillar of Geryon is a column on which is a bas-relief of the layer's former ruler. It's missing its head and one hand. Stick your hand in the hand-hole, and it gets cut off — but then replaced by a slightly rubbery replacement, which counts as a magic weapon and can slip through weaknesses in enemy armor (and also tempts you towards lawful evil). Put your head in the head-hole, and...
Knowledge DC 25: No one who has given her head to Geryon has ever gotten it back.
Pre-3rd Edition, D&D had an entire class of magic item meant to serve as Schmuck Bait: Artifacts. (As of 3rd Edition, many are just really powerful magic items, though some retain their legacy of Gygaxian cruelty.) The whole idea was largely inspired by things like The One Ring — complete with incredibly specific means of destruction — so that should probably tell you what a campaign was meant to be like with one around. The aforementioned Deck of Many Things is one of the most obvious and well-known ones, and people keep falling for it to this day, along with famous ones like the Hand and Eye of Vecna. Players who should be smart about this keep getting suckered in.
Any Call of Cthulhu game is inevitably Schmuck Bait for anyone with passing knowledge of Lovecraft's work. Why read the fabled book of dark magic? Why go into that creepy cave? Because otherwise the game would be over. Specifically Horror on the Orient Express takes this to ridiculous levels. A suspiciously well-informed quest-giver tells you that you must save the world by collecting the six parts of an ancient doomsday artifact, all located conveniently on the path of the Orient Express. Inevitably, the Genre Savvy players ask themselves why they're re-constructing a statue that can destroy the world when the individual parts are dangerous but not cataclysmic. When you finally reach the end of the line after fighting cults, vampires, and fascists and suffering character deaths, permanent insanity, and increasing stat penalties as you collect the parts, it turns out that the quest-giver was the Big Bad all along, and just wanted some Unwitting Pawns to collect the parts for him. Then again, the alternative to taking the Schmuck Bait is to just not take the job, in which case you sit around and stare at each other until the Keeper can come up with a new adventure.
The Computer strongly advises Troubleshooters not to press any buttons labeled "Do Not Press". During the course of this mission, you may encounter buttons incorrectly labeled "Do Not Press" by traitors. Troubleshooters are advised to press these buttons. The Computer is your friend. Trust The Computer.
In one of those after-The-Computer-crashed adventures that no longer exists, The Computer was infected with some ancient evil. After the players manage to trap it, it is stored in a box labelled "Do Not Open". The schmucks in Alpha Complex open it, and unleash the ancient evil (again).
In another official adventure, one of the Troubleshooters' secondary assignments is to test an experimental "Traitor Killer". When you pull the trigger, it explodes. This is intentional; the assumption is that there's a traitor in the team, and that he'll volunteer to test it rather than let a teammate use it against him.
Some Warhammer players will do this often either by having a couple high value targets for the enemy so they could focus their all their attention against it instead of the real threat until it was close enough to do some damage.
Others send in fast cavalry or flying units to get just close enough to Night Goblins so they release their fanatics or units with frenzy in general and send them on a wild goose chase. Cue Benny Hill theme.
In Warhammer 40,000, this is a well known psychological tactic called a Distraction Carnifex, named after the Tyranid living siege engine, where you wave a very scary model in an opponent's face while the much more deadly but lower profile units do their job, ex: a Possessed Chaos Vindicator goes full speed towards the enemy with their BFG while some Predators sit back and eat your opponent's tanks with their Lascannons. One can also you cheap, weak, and very obnoxious units (aka tarpits) to fufill the role of bullet sponges such as Nurglings, small fat daemons that sneak up to the closest cover and attack, or Ork boyz supported by a Weirdboy, who uses Da Jump psychic power to jump them close to the enemy and not get chewed up when footslogging.
Parodied in Munchkin with the Duck of Doom. "You should know better than to pick up a duck in a dungeon."
In Legend of the Five Rings, the Tomb of Iuchiban was constructed to seal in the deathless, body-jumping sorcerer. The entrance requires several heavily guarded artifacts to open, and every room is full of deathtraps. Incredibly powerful wards and spirits prevent anyone from simply tunneling or using magic to bypass the traps. You may be wondering why the tomb has an entrance at all: Why didn't the builders just seal the tomb on all sides with these wards? They did. The "outer tomb" network of traps is there to lure in Iuchiban's followers and kill them off. There is no way to access the tomb itself.
In the official Mutants & Masterminds module, Time of Vengeance, set in the Freedom City universe, there is a point in the story where Mr. Infamy offers you his card for a chance to get enough power to defeat the villain, no strings attached. Except, of course, power corrupting, and corrupting absolutely...