Elemental spell fumble should be different from elemental spell failure. Personally I've devised a mechanic for determining where the ball/bolt goes as opposed to where it is intended to go, if a spell is fumbled. For instance, rolling the fumble roll, a ball will explode on its way to the target equal in distance to the roll as a % of the way. High rolls (100+) means it literally explodes right away (in your face). For bolts, I use a d6 or d8 to determine direction from original aim, depending on wether the map uses squares or hexes, and then roll a d10 for # of squares off. This is the new "target". The bolt impacts on the first obstacle on the way to the new target. As with balls, detonation on caster is possible, and 66 on a fumble does something bad (like hit closest ally)