Add my testimonial to the list. Has been happening to me too, in FV, Tim Deep at aviaks, Dreadlands. No failed message, mana IS definitely burned. The only cause of this I can gather, is that it's up/down positioning causing this. Even with the mobs in plain full view, if the area is not 100% flat - just a slight incline gives this a chance to happen. Either me being slightly higher than the mobs, or the mobs slightly higher than myself, doesn't matter.
EDIT: Also happened in Burning Woods today with hornets
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