Here's a fairly critical bug:
Anytime you attempt to switch or replace a skill on the skill bar when the game is paused, you can make the switch successfully. However, when you unpause the game, the skills revert back to their previous order. This means you can't reorder or replace your skills. This happens to me after having rebound the skill keys (I'm not sure if it happens when they are on default).
Also, it is impossible to delete skills using the DEL key if the game is not paused. If you delete them when the game is paused, they reappear once you unpause.