I have a variant for spell backfires associated with metamagic use.
In essence, metamagic doesn't increase the spell level you prepare it at, but it increases its instability. The base chance of failure is 10% per spell level that would have been added.
Thus attempting to cast a silent magic missile has a 10% failure chance.
Attempting to cast a quickened, maximised magic missile has a 70% failure chance (+4+3 x10).
If a spell fails there is an equal probability of the spell backfiring. If it backfires the result depends upon the spell.
damaging spells ground zero at the caster.
other spells fail in an unusual or entertaining fashion (or do 1d6 per spell level damage to the caster, as per scroll mishaps).
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I didn't include sorcerers in this consideration - if I did I would allow them to cast as a full round action at the normal percentage failure, or as a normal action for double the failure chance
I hope this may be helpful!
Cheers