Arena seems to think that I have 3 Marks of Darkness-- but I should only have 2.
Normally, with a 30 card deck, 2915 cards unused.
In arena with a 37 card deck, 2909 cards unused?? The extra card can only be that third mark, but I have no idea how it just popped in there.
Edit:
I've reproduced the bug by creating a new arena deck. I re-added in all the cards of my original deck, including only 2 marks. I then submitted the deck, and then went to modify it. In the second arena creation/modification screen, the extra Mark of Darkness appears despite the two real ones already being in the deck.
Edit 2: Removing all 3 marks from deck resets available marks to 2. Removing only one mark out of 2 still adds in the extra mark.