This is where I currently am regarding insanity as a result of spellcasting.
Everyone has a "mental fragility" or just "fragility" rating, expressed in a number of ranks. The more fragility ranks you have, the more quirky you will be under normal conditions, and the easier it is for events to push you over the edge. Note that events having nothing at all to do with magic can increase your fragility and/or push you over the edge. The first time you deal with a death, the first time you encounter an undead monster, anything that your brain wants to refuse to process as "the world is not supposed to work like this!" Even a bad breakup of an emotional relationship can do it, although thankfully that's pretty rare.
But specifically to spellcasting, this is what I currently have regarding a fumbled casting. In all but one case I'll be using Rasyr's labels from "The Ecology of Magic." Note: "Fortean effects" are defined as random minor magical effects. Rainbows, a tiny thunderstorm right over your head, doves flying out of your butt, whatever. Also note that "Anchored mana" is mana trapped inside of physical objects, eg spell components. The following tables apply in 50% of all casting fumbles, the other 50% follow the normal rules for the game system you're using.
Filtered mana:
01-40 You are aware of everything. Spend 1-5 rounds coming to terms with the fact that you can’t possibly process it all, before reluctantly returning to reality.
41-70 Random Fortean effects in a d100/2’ radius. Possible minor damage to those in the radius.
71-90 You are now subject to giggling fits and occasional hallucinations. +25 to attempts to cure you. Add 1 Fragility rank.
91-00 You suffer from delusions. -25 to all attempts to cure your insanity. Add 2 Fragility ranks.
Rendered mana:
01-40 50% chance to become catatonic for 1-10 rounds any time you roll UM 01. Permanent until cured. +50 to attempts to cure your insanity.
41-70 Random Fortean effects in a d100/2’ radius. Possible minor damage to those in the radius.
71-90 You are unworthy of your God. You aren’t even worthy of your party members. +25 to attempts to cure you, if they can get you cured before you commit suicide. Note that whether or not this is actually true is beside the point, you still believe it. Add 1 Fragility rank.
91-00 You are The Ultimate Avatar Of Almighty Gawd™. Let’s hope your God agrees with you. -25 to all attempts to cure your megalomania. Add 2 Fragility ranks.
Personal mana:
01-40 50% chance to panic for 1-10 rounds any time you roll UM 01. Permanent until cured. +50 to attempts to cure your insanity.
41-70 Random Fortean effects in a d100/2’ radius. Possible minor damage to those in the radius.
71-90 Paranoia. +25 to attempts to cure you. Add 1 Fragility rank.
91-00 Multiple personality disorder. -25 to attempts to cure you. 50/50 chance for each of those personalities to be paranoid, 10% chance to be violently so. Add 2 Fragility ranks.
Anchored mana:
01-40 You now have Tourette's Syndrome.
41-70 Random Fortean effects in a d100/2’ radius. Possible minor damage to those in the radius.
71-90 You have acquired a random phobia. Roll vs. SD at -25 to act rationally in its presence. +25 to attempts to cure your insanity. Add 1 Fragility rank.
91-00 You are nothing but a quivering, crying little blob of goo. -25 to all attempts to cure you, not least because you’re terrified of the doctor. Add 2 Fragility ranks.
My current take on Fragility is that all spellcasting uses the random roll, plus your skill in the spell, plus one per rank of fragility. In other words, the crazier you are, the more likely you are to have your spells work. Your ability to choose the right target may suck, but wow your spells work! This goes along with the rationale that you have to be temporarily insane to cast spells at all, and also does the GM a favor by giving him an endless supply of "monsters" in the form of spellcasters who have gone insane, and who are more deadly than ever before as a consequence. It also challenges parties to find ways to keep a crazy spellcaster on tap, and just turn him loose when needed. The bad news is that all the risks associated with caster insanity also apply to your healer.
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