When I spend skill points, hitting the wrong button is somehow more annoying than fat-fingering into an ogre. I'm sure I'm not the only one who irrationally feels this way.

Also, even when you have a manual, you're going to want to know how many skill points it's going to cost to get to another plateu for many skills; so it should be possible to see what you could get by spending skill points, and then change your mind. Suggest: hitting "z" should commit skill point expenditure, while "Z" (or Esc?) should cancel the skill points you just pretended to spend.

EDIT: This cancels my previous suggestion of letting you unspend skill points one at a time.