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What are the most lethal raid mechanics?

blizz -> wysłany:
(.. in your opinion.)

What are some of the most dangerous mechanics you remember from bosses that constantly wiped your raids?

Here's a few off the top of my head.

Living bomb from Baron Geddon. The classic noob-check ability that wiped many of my MC raids. My RL eventually got so sick of people messing this up that he kicked a new guy on his first day due to failing it.

Iskar's phantasmal winds. Please pass the eye. Hey, could you pass the eye? I'm falling off, could you pass the eye? I really need you to pass the eye to me. Seriously pass the eye. Hey dude pass the eye! I'm about to die unless you pass the eye. PLEASE FOR THE LOVE OF GOD PASS THE EYE. Enough said.

To clarify, we're not talking about bosses as a whole. Just single mechanics that kill a lot of players.
blizz -> wysłany:
Every boss that "required" any kind of semi-well-timed movement, basically.

SSC/ICC - Elevator Boss
Kara - Flame Wreath, Shade of Aran
DS - Roll, Spine of Deathwing
MSV - Floor, Elegon
ToT - Maze, Durumu
BRF - Trains, Operator Thogar

Personal favorite, though, was always the people who had no idea what they were doing on Yogg-Saron (It never failed that 1/3 of the raid had no idea what to do with most of the mechanics). A good way to drive both the raid and myself insane a few times a month. @_@