Well, the only fair kind of game to do these in is in a total switch to defense(so all attacks use attack rolls instead of saves), and of course, for the fumbles to be interesting, but not critically fatal, a mistake many fumble charts make(the consequence being completely out of proportion to the attack attempted).
Robby's confirmation roll idea adds a safeguard as well, though I'd preferred simply failing the required target by 10+, indicating gross incompetence from the start, and ensuring that all consequences are scaled to the attack/task attempted.