Fumbles happen on a roll of 1 on a D20
Fighter – Weapon drop. You have butterfingers. You got to pick it up next turn.
Paladin – Hubris. You boast and make a flamboyant but ineffective attack. -2 AC this turn.
Ranger – Bird watcher. Is that a rare bramble-toed wood owl? -2 to all rolls next turn due to distraction.
Cleric – Momentary lapse of faith. Why would a good god create a world with orcs and zombies? No spells next turn.
Druid – Roadkill. You have stepped on a bug or rodent or lizard or something. You feel guilt and shame, and a momentary disconnect from mother earth. No spells next turn.
Magic User – Memory lapse. Roll random spell. 50% chance your forget it, 50% chance you accidentally cast it while sneezing.
Illusionist – the Job. One of your memorized spells goes off at random.
Thief – Wallet drop. Drops an item at random from his inventory.
Assassin – Go boom. You fall to the ground and make a loud crash.
Monk – Finger jam. Your attack jams your wrist or finger or toe or something. Take D2 damage.
Barbarian – Verbiage. The barbarian says a word with 3 or more syllables. Any party members within 20′ must save vs. paralyzation or receive -1 to AC for the rest of the turn.