Yes, we accidentally the Kologarn hard mode achievement!
Kologarn 10man over the weekend was accidentally done with the hard mode. How was this managed? Pretty easy actually - easier than you would think.
While you are downing the right arm, make sure there is a goodly dose of AoE going on and splash damage onto the left arm accumulates. When you are ready, and the left arm is at about 10% or less, smash the right arm, then smash the left arm, ensuring that smashing the left arm kills Kologarn. Collect your Hard Mode achievement which isn't actually very hard. I suspect that this will get changed soon so that it wont apply.
I suspect this method will work in 25man as well with a bit more management, perhaps save a heroism for it.