I was in an instance with some guildmates and was trying manaburn on for size. I waited for the first named (heroic) to get to less than 50% and queued up Manaburn. It cast. I used all my mana. Zero damage.
I figure this is a fluke of some sort. Go to do the same on the next named. Same thing.
It seemed to work on the rest of the heroics in the instance, but not the named. What am I missing?