That I faced, Defile, sweet fuck defile. Or maybe Illidans eye beams, I saw him cleave through 10 80's with that once.
Lots of mechanics are "unforgiving" if not done right. In fact, most of them are, during progression.
What the thread is really about is "the most unforgiving" and its ratio to "how easy is it to fuck up?" IE, something that's really unforgiving but requires being brain dead to fuck up (IE, the debuff on H-Spine) is not unforgiving, imo. Something that's easy to fuck up but won't wipe the raid necessarily (Archimonde) is closer, maybe.
To me, the ultimate easy to fuck up + ridiculously unforgiving were:
Wrack as a healer on Sinestra (I still have math nightmares of trying to calculate how much the damn debuff had left on it)
Frost orbs on H-Ascendant Council
Seeds on H-Ragnaros. Sons are unforgiving, but any coordinated raid group should have been able to do it relatively easy, same with the triangle breadth of frost. Seeds could just plain fuck you up if you couldn't move and do massive AoE at the same time as a raid, if you followed the Paragon method.
Close traps as a tank on H-Ragnaros, p4. Having to kite Ragnaros way away from the breadth so he didn't get trapped too early was scary times, I'll tell you.
Defile is an honorable mention, but again, it seemed like if you had a group of competent people it went away pretty quickly as a problem.
H-Nef interrupts when both your primary and backup interrupt got bombed was interesting. The third person was usually kind of unreliable and/or a healer and pretty busy healing.
Of course, C'thun's eye beam was ridiculous.
Defile, definitely. Also for a more silly one the elevator in BWD, our tank died to it so many times its not funny, he'd literally just fall through the elevator.
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Bdk Nagrand / Astae Nagrand
Pokemon X FC: 4656-7679-2545/Trainer Name: Keno
"And WATCH THE TAIL!"
Also, for some reason Twin Emperors came to mind, especially if you chose to not use a warlock tank and instead depend on battle-shout spam.
"Don't walk behind me; I may not lead. Don't walk in front of me; I may not follow. Just walk beside me and be my comrade." - Albert Camus
Oh this question. I just had to laugh and think about:
"Who didn't use his tear?"
Archimonde's Airburst in my opinion.
I'm going to have to go for Gorefiend's Construct Mechanic. Defile is unforgiving buy not too difficult to avoid. In my BC raiding guild you were not allowed to come to Gorefiend unless you have completed an online flash game designed to mimic the ghost controls in defeating the 4 constructs.
This may not be a boss mechanic but learning the DPS rotation for a BC Arms Warrior was mind numbing for anyone who have experienced it. Timing your Mortal Strikes and Slams with your white swings was near impossible without the help of Add-ons.
This is the most Unforgiven boss in World of Warcraft.
Most of the unforgiving mechanics have been mentioned but not Hour of twilight on ultraxion 25hc. While progressing with semibad gear and people needing to really min/max there were waaay to often some dps tunneling the boss and trying to exit the realm at 0.3s or something and having a latency spike. "sorry, laggspike" was said on average 2bilion times per pull on ultraxion. simple mechanic, but raidwipe if someone in the rotation forgets/ fucks up.
mhhh things that come to my mind (cant say anything for pre-cata)
frozen orb on ascendant council
baleroc heroic debuff
ragnaros sons of flame, meteors, empower sulfuras
Since about 99% of this thread is about Defile, I'll go for the one that personally broke me as a raid leader. I never truly recovered from it.
Archimonde. Fucking Archimonde and his stupid sodding air burst, where he knocks people up into the air. Although I suppose it's not strictly that ability (though it was the one people most screwed up on); it was the one where every time someone died, Archimonde gained a charge to use a super raid-wipey ability. Which then likely caused another death, so he gained another charge, then another death, etc. etc...
Just. Fuck Archimonde. Fuck him all the way to hell.
Archimond. One person failing to use their tears or running through the fire started a domino effect of people dying.
You are not your role. You are not how much gold you have on your account.
You are not the mount you ride. You are not the contents of your bank.
You are not your epic purples. You are not a special and unique snowflake.
You are the all-grinding, all-farming crap of Azeroth.
As many have mentioned and I think we'll see it to be a clear winner, I vote for defile, especially on heroic. Whenever he casted, I pooped my pants a bit.
By the way while I may sound complaining (and I am sortof) I have experienced much, much worse. Did Blizzard pay BioWare to show us MMORPG players how bad server lag can be? Geez, SWTOR was horrible in this regard.
Madness was also strange DPS-wise. Doing enough DPS (and later too much DPS) on Madness was a funny one. Eventually in 25m you could just let tank die from impale and res. In 10m you could only do that once and had to literally stop doing any damage for like a minute. And the times where the DoTs ticked too much on Deathwing, or the AoE damage was too low. Then some of the adds started to dance on the tentacle and I thought I was drunk.
"When i am done with you, you won't trust your own mind."