A lot of people have commented on my deadly fumble chart that I put in another thread (tried to put a link in but it kept coming in up paizo homepage)
Multiple people said that I’m punishing the melee fighters way more than casters so it got my thinking, what if a caster could fumble a concentration check and if they do they could have something bad happen to them. Maybe they provoke an AOO unless you guys have a better idea for the punishment. More help please.
Maybe they could lose 1 spell per point of damage caused to them by their spell misfiring on them. This could be dangerous if your magic user likes damage spells. It could be really funny if the summonses turn and attack the caster.
Is this sortof what you were asking for?
Can a bad roll fumble any skill check or just a concentration check with your rules? Even then, a spell caster isn't going to be making checks to cast every spell so he's still not as open to fumbles as the melee fighters.
All that being said, how about either:
dazed for d4 rounds
stunned for one round