Not sure if this is a bug, or just an odd UI decision, but I feel that it would make more sense to display the 10th ability on the abilities hotbar, instead of just 1 - 9. What's especially odd is that the 10th ability can still be used by pressing the ability 10 key (default 0), but switching to the next hotbar puts the 10th ability in slot 1, effectively causing it to take up 2 spaces in your total hotbar space.