YMMV / Witchery

• Game-Breaker: Vampiric Poppets. If you make four or five of them and bind them to something hardy, such as a few iron golems, you essentially gain several extra health bars, making you extremely difficult to kill. They'll take up a few spots in your inventory since they don't function when in Poppet Shelves, but that is a small price to pay for near-invulnerability.
• Goddamned Bats: The mandrakes in Witchery. Anytime that a fully-grown mandrake plant is harvested during the daytime, it spawns a mandrake mob that sprints around making high-pitched shrieks and causing the nausea status to all nearby players. Between the nausea and the creature's size and speed, you'll probably tear up half your farm trying to kill it to make it shut up, though it cannot actually harm anything. No one mourned when they were not implemented in Bewitchment.
• Paranoia Fuel: Curses. At any time, another player can perform a relatively simple ritual to inflict some kind of negative, and sometimes crippling, status on you, ranging from temporary blindness to a wasting disease that will almost certainly kill you again and again until it wears off. The worst part is that unless you use a Crystal Ball or are familiar with the various curses, there's basically no way to know that you have been cursed and all of this will seemingly be happening for no reason whatsoever. Probably most notable for this trope is the Curse of Insanity, which causes a player to hallucinate various threatening noises nearby, including enemy vocalizations, a creeper preparing to explode, footsteps, and an arrow being loosed. Even if you figure out that you've been struck by the curse, it's near impossible to know which of the sounds are real and which are fake, meaning that you'll always be checking over your shoulder just in case.