When at the level up screen it does not appear one can cancel it and return to the game (no cancel button, Esc does nothing, etc.).  I presume this is a matter of interface design, not a bug.

 

Being able to cancel is very useful for when wants to see what is available at the next level without committing to leveling immediately.

And if you forget what type of weapon you had equipped and wanted to get skill for <_<

I noticed this as well. Another bug I've encountered in the level up screen was when I gave a point to a skill my character's clothes became transparent (Aumaua Fighter)