Not one of the unachieveables, but the secret "The Destroyer" achievement must be at least somewhat glitched. I have played and won over 150 Arcade Mode levels and no achievement. I had heard before it's glitched to some extent, and this must be some proof of that.
50k achieved 10/10/2011
100k achieved 7/3/2014
Latest: Assassin's Creed Chronicles: China