DAMMIT DAMMIT DAMMIT

HOUSERULE - SPELLCASTING
Wanted an alternate to automatically losing the Spell if struck during casting.  These rules allow the possibility of completing the Spell despite being hit, if the Player chooses to take the risk, and also provides for a chance of a more severe penalty to occur...

If caster is struck for damage prior to starting the Spell, the
Spell cannot be cast, but is not lost.  If caster is struck after beginning, but prior
to completing a Spell, a save vs Spells may be (Player option) made to determ if casting may be completed (-1/Spell lvl).  If failed, the spell is lost, and if the fail result is "1" or less, caster is subject to the Spellcasting Failure Table.  Damage greater than 1/2 of the Spellcaster's HP negates the option of saving, Spell is lost, no other effect.

SPELLCASTING CRITICAL FAILURE (d6)
1. Spell attempted affects caster, and possibly others
    in area of effect (save, if any, applies)
2. 1d4 additional random Spells memorized also lost
3. Auto Stunned 1d4 rnds
4. Unable to employ Spells for 1d4 trns
5. Unconscious for trns = to lvl of Spell attempted
6. Feebleminded, save -4, permanent until Dispelled, a successful 
    Save requires a second roll on this table, using a d4

Athanor has the best take on this I've seen, I just wanted something less extensive.