These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The "Staff of Napalm" and "Chainsaw of Bloody Dismemberment" cards.
Demonic Spiders: Anything that prevents you from using your hard-lootedearned equipment against it. This becomes really bad when someone notes that you're just winning and sends over Squidzilla to say hello.
Game Breaker: In the original Munchkin, a thief with some good luck (or some Loaded Die cards) could quickly become the most powerful player in the game. The Kneepads of Allure — which the thief can steal for their own use — allow you to force somebody to help you with no actual reward, making you as powerful as yourself and the next most powerful player in the game. (The Kneepads have since been Nerfed.) Both of these are countered by the rest of the players pretty much joining forces to rob you of your powers.
Technically, nothing in the game is really a game breaker since the whole point of the game is to win by being a Munchkin.
Manipulative Bastard: The best players are always these. Examples of Munchkin bastardry include: "protection rackets" in which a player prevents another player from being able to fight a monster, then offers to sell them defensive cards for a high price; lying to newbies about what their cards do/are worth; and asking a player to help you gang up on another, only to backstab them later.