Since I just posted a couple of class variants which call for spell fumbles, I thought I’d post a table for such things. Like most people, I haven’t found one I like, so I cobbled my own together from a bunch I’ve found across the internet. (I’m being very original lately).
01-05 No effect (spell simply fails)
06-15 Minor backlash of arcane power. You are dazed/dazzled for 1d6 minutes.
16-30 Spell hits a random target within 100′ (or the AoE is centred at a random location).
31-40 You are wreathed in unearthly fire; take 3d6 points of damage.
41-50 You believe that the spell has been cast normally, and are dazed for 1d6 rounds.
51-60 A minor-level demon is summoned, and attacks you.
61-70 You are forced to glimpse into the Beyond. You take 2d6 points of subdual damage, are stunned for 1d10 minutes, and must make a WIS check or gain a random madness.
71-75 Moderate backlash of arcane power. You must save vs. spells or suffer a critical failure each time you cast a spell for 1d6 minutes.
76-80 Minor spell storm. 1d10 random spells (of level 1d3) are targeted/centred on you.
81-84 A medium-level demon is summoned, and attacks you.
85-90 Your blood begins to freeze in your veins. You must save vs. spells at -6, or take 2d6 points of cold damage, and then save again each round at +1 until you make the save.
91-94 You arm is shattered by the backlash; take 2d6 points of damage.
95-97 Major backlash of arcane power. You are dazed for 1d6 rounds, and cannot cast any arcane spell or use a magic item for 2d6 hours.
98-99 Major spell storm. 1d10 random spells (of level 1d10; roll twice on 0) are targeted/centred on you.
100 You explode in a shower of gore and energy. You are dead, only to be resurrected by a true resurrection or wish. All within 30′ must take 6d6 points of force damage, and make a CON check or be nauseated for 1d6 minutes.