It might not be the most serious bug in the world, but it's definitely annoying. Sometimes I log onto a character and discover that the mundus stone passive on them has disappeared since the last time I played.
The most recent example was that the mundus disappeared on a character that had just reached level 50 (Imperial templar). I went to give her another mundus, and the game wouldn't even let me click on the mundus stone. Rather, it would let me click, but the button didn't work and wouldn't let me give her a mundus again. These are definitely buggy.