Defile, Shadow Trap from hc lich king and World in Flames, Lava Wave on hc ragnaros, best mechanics to spot the baddies in your raid group.
failing to kite raggy hc in p4 = instant wipe. tho that's an ability dependant on ONE persons ability to get the fuck away. defile was dependant on every person in the raid knowing what to do, which is what has made it the ultimate ability.
Defile, by far. Out of the 400+ wipes it took to kill the bastard on 25HC this accounted for more than half the tries. You needed 20+ people able to run out fast. It wasn't on 1 person, it wasn't 100% guaranteed you had to run out which made it dangerous cause you always thought/hoped it wasn't you. You also had your usual suspects, the ones you prayed wouldn't get it.
In Terms of a Single Skill that wipes the raid:
Geysir of pre Nerf Ragnaros HC > Defile of Lich King HC.
Standing a few yards wrong caused the Geysir and in 99% of cases it was a wipe... after having cleared the previous Phases for 10+min
Defile was hard too, but to really wipe the raid several players had to be in the wrong place and it was generally avoidable with good raidmovement and if people used their gapcloser.
From newer fights Unseen Strike from Blade Lord Tayak can be unforgiving at least in 10man if you don't have a Priest. 1 Player standing wrong can easily cause 2-3 other players' deaths. But as the fight is generally quite easy it doesn't have that much of an effect on the total difficulty.
Bosses with unforgiving mechanics in general:
1. Ragnaros HC... every Phase had a Instant-wipe ability if something went wrong. Fire Trap activated too early in P1? Wipe. ONE son not stunned properly and reaching the hammer? Wipe. Running 1sec too late before Seeds spawn in P2? Wipe. P3 not enougth DMG? P4 Geysir? P4 bad roots-spawn? P4 bad Dreadflame spawn? Wipewipewipe.
2. Lich King HC... pretty similar to Ragnaros.
3. Muru Prenerf... mechanics were simplier back then but still they were hard. One player doing a mistake from pulling aggro to standing in the wrong spot most of the time caused a snowball-effect which lead to a wipe a few seconds later.
Zian's Shield of Darkness. One hit and your raid wipes.
I was a newbie human rogue adventuring through elleyn forest when I got this game way back in the day........I ran into a gnoll called hogger.....I thought I could attack him then run away if he was to hard........when he started chasing me..... that was the hardest most terrifying npc mechanic I came across......NPCS CAN CHASE YOU?....NOOOOO RUNNNNNNN!
`edit` well either that or.....this http://www.youtube.com/watch?v=oTuui9lg0Ro Again hogger but buffed to hell through the joys of blizzcon.....poor premonition lol
-Doing too much DPS on a claw/wing on Madness of Deathwing, causing it to spawn blistering tentacles prematurely.
-Idiots passing the pheromones back and forth on Garalon.
-Twilight Onslaught spawning in the bugged fire on Warmaster (no one to blame here, it just sucked and i hate it).
-Tainted Cores on Lady Vashj.
-Windbombs on Mel'Jarak.
-The dog on Shannox that required you to hit it with your hardest nuke in next 1-1.5 seconds or it would rape somebody (Rageface, i believe).
-Zian's Shield of Darkness.
-Any kind of Living Bomb (you are the bomb, get away).
-Torment, Tormented, and Countdown on heroic Baleroc (my personal favorites).
The dude that had to be mage tanked in the High King Maulgor encounter at Gruul's Lair. All it takes is one hunter to fire off an Arcane Shot (which used to dispel magic) and the mage can't spellsteal, the mage dies, the boss starts flinging spells at everyone in the room and you all die.
But, seriously, I'd say the most wipes I had while raiding were in phase 2 of Heroic LK. It wasn't necessarily because Defile is "hard". But when you have some derps that love to drop it directly on top of LK...
I mean, it wasn't as hard an encounter - but then again, Flame Wreath was the only hinge on it. Other than that, it wasn't even tank & spank, it was just spank. But the mechanic was the most unforgiving.
as far as current tier go, people not moving out fast enough with arcane resonance on feng can nuke the whole raid in a matter of seconds
I was going to just reply with what I thought, Defile on Lich King.
Seems as though I was beaten to the punch by 90% of the posters here.
Defile..... I hate you.
You have caused so many wipes, even at level 85 in LFR DS gear i remember idiots failing at it cause us to wipe. The night the openraid group (run by a outstanding RL) did HC 25 LK and both achievements at the same time was amazing. 2 wipes and then we did it flawlessly.
Servers PUGs and other groups..... not so much.
Defile is one of those "If one person screws up you wipe" mechanics that makes fight extremely annoying and fun at the same time.
"Humility defeats pride, Master Yang has preached. Pride defeats man"
Defile wins in my book.
There have been other troll bosses too (Yogg no keepers sucked, Mimiron was a freaking troll robo gnome) but Defile was a double edged sword. One it caused you to wipe. Two it made you listen to Tirion and Bitchking RP for an hour every single fucking time. So it was like a raid wiper with a punishment where you would wake up in Goldshire/Barrens chat every single time.