In combat, can fumble an initiative, attack or defence roll. Cannot fumble damage or soak (in both cases, it is treated as a simple '1').
If fumble initiative, cannot make an action that round, though can declare defenses. Since initiative is only done at the start of combat, this only affects the first round of combat.
When fumbling an attack roll, one of the following could happen:
Normally, the attacker chooses one of the above options. If the defender has a greater skill (*not* ability), then the defender can choose. In all cases, the attack is a failure.
When fumbling a defence roll, one of the following could happen:
Normally, the attacker chooses one of the above options. If the defender has a greater skill (*not* ability), then the defender can choose (yes, this is subtly different from attack fumbles, in that it is not the fumbler that gets to choose by default).