Holding down and continuously attacking with Blade Fury displays odd mana consumption, it consumes a lot of mana (much more than it should consume) but then it “rolls back” some of the mana cost and your mana bulb refills a bit.
Here’s a video somebody made of the bug.
Similar thing happening with Throwbarbs, wondering if it has something to do with CtC
Video only available to subscribers. 
I tested Blade Fury with one of my single player characters with no gear and I had the same issue, so I don’t think it’s related to CtC at least in BF’s case.
there we go, should be fixed… didn’t realize twitch changed my settings -_-