Negative experience required for level up / turns elapsing
A low-level character with a few experience points missing from a level-up, standing at the entrance to the puppy cave. When entering the puppy cave, a message appears that some experience points have been gained (for locating the cave) - at this time when checking the experience window with x, it shows negative amount of experience points required for level-up and the level-up actually happens when moving away from the stairs.
I think this might have something to do with turns elapsing as the character had the long stride talent. Maybe entering the dungeon did not take "a full turn" because of the lowered energy cost of moving and that caused this to appear. It would also be interesting to test if this can cause other problems (for example saving "in the middle of a level-up").