In addition to their ordinarily allowed spells, wizards may choose any one extra spell of any level and designate that as a "poorly remembered spell". This spell represents the caster Fucking With That With Which One Ought Not To Fuck.
This is a spell that the wizard kinda mostly is pretty sure he saw someone cast once while he was leaning over somebody's shoulder and pretty much thinks he can kinda guess how it works.
When cast, this spell has an unmodifiable 50% chance of working backwards--striking/affecting the caster and/or her allies (if it's a harmful spell) or the caster's enemies (if it's a beneficial spell). If the spell isn't strictly beneficial or harmful, then the GM is encouraged to interpret "working backwards" in the most disastrous possible light.