When a character's failure is exceptionally bad, he or she can fumble. Skill fumbles usually are directly detrimental to the intended purpose. Skills which involves objects that are fumbled often destroy the object in question, while skills meant to heal or harm an individual may have the opposite effect. Spell fumbles, on the other hand, are spectacular occurences that have a dizzying variety of effects, usually based on the type of spell being cast. The possibilities of a fumble aren't nearly as noteworthy as the doom they call down upon the unfortunate sods who cast the errant spell.
TODO: Less vague explanation.