When the call to sacrifice came, a player who had not yet sacrificed (or been killed at all) during the raid that day clicked the big flag that spawned, but nothing happened. Then a few seconds later, we got the failure emote and multiple players died. This was reported by 2 different players, the last 2 consecutive weeks. These players have successfully negotiated the sacrifice mechanic in the past, so it isn't like they are just floundering around unsure of themselves. Not sure if the timing is important, but both times it was after several successful sacrifices when the Avatar was at about 20% health. "Like" if this has happened on your raid!
What Nniki said … ya gotta die. The only times we have noticed problems on this on Live is when player life saving aas/spells kick in. Know what classes have passive abilities and never have them click. And make sure those that get buffs, know to click them off. Or just be brave and always go for No Sacrifice cheese. Despite what people tell you, everyone loves a mechanic where they die without warning. Edit - I do believe we had one fail where we took a bad lag spike just as someone clicked and it didn't work. You didn't mention lag so probably not an issue you saw, but I'm pretty confident about my recollection.
I suppose is possible that in both cases there was a DA or something that triggered, but generally we are aware of that issue. I guess we'll just have to keep trying and see what happens.
Also that - There is an emote that filters to the "Other" filter along the lines of "The Restless Avatar of War accepts the sacrifice."
The ach will also fail if you super DPS the event and try to blow thru that mechanic. There's a second life tribute that can be run that will fail the achievement if it kicks in, as a bonus there is no way to check if someone is running that tribute aside from trusting everyone to not have it on.