Spellcasting Ability on page 254 has no mention of how to treat fumbles, on a fail no mps are lost. So it's your call. In my games we play that if you fumble, you do loose the mps. However MGF on page 6 dictates that where appropriate funny things can happen if it furthers the game. I've had disruptions bounce back and hit a donkey, ignite setting fire to a nearby cart the players were guarding, etc. Have fun.