Here is my attempt to create a "reasonable" fumble system.
First let's clarify the terms I use for this system. (If these differ from the official terms for the same concepts, or if they strongly clash with the standard rules in some way that I have misremembered, please let me know so I can retrofit these rules to comply with the standard)
* Normal attacks: the single attack action everyone is normally allowed to make in an encounter, plus any iterative attacks (additional attacks gained by having a base attack bonus of +6 or higher)
* Natural attacks: attacks with natural weapons
* Offhand attacks: any additional attack actions granted by virtue of using the two-weapon fighting rules, including attendant feats, and excluding multiple attacks from natural weapons
* Extra attack: any other additional attack actions granted by class abilities, feats, spells, etc., excluding attacks of opportunity and two-weapon fighting rules
* Attacks of opportunity: rules as written
* Final attack: A character can have multiple "final attacks", for example if they are two-weapon fighting
Rule #1. A character incurs a fumble threat with an attack roll if:
(a) a natural 1 is rolled for an attack roll, and
(b) The attack roll is for the final (or only) normal attack he is entitled to make on his turn, or
(c) The attack roll is for the final (or only) offhand attack he is entitled to make on his turn.
A character does not necessarily risks a fumble with the last attack he actually makes; if a character is entitled to three normal attacks but only uses two of them, he does not risk fumbling that round. Characters engaged in two-weapon fighting can be at risk of fumbling with two attacks, their final normal attack and their final offhand attack.
However, a low-level character with two weapon fighting--one normal attack and one offhand attack--would be at risk of a fumble with both attacks.
Rule #2. Extra attacks and attacks of opportunity, because they allow a character to push beyond his normal limitations, are always at risk of a fumble.
Rule #3. Natural attacks never risk a fumble unless the creature has lost their Dexterity bonus to AC, in which case it is at risk of a fumble on its final (or only) natural attack.
Rule #4. Fumble threats are adjudicated similar to a critical threat. Following the natural-1 roll, the character must make a fumble check: a d20 roll with the same bonuses and penalties applying to the previous attack against a DC of 20. Success converts the fumble threat into a normal missed attack. Failing the DC results in a fumble. As expected, a natural 20 always succeeds, and a natural 1 always fails (and incurs the threat of a critical fumble).
Rule #5. A fumble event can take one of several random forms, appropriate to the circumstances of the attack, the weapon used, and the intended target of the attack. Roll d% on the following table, or use any other list of fumble events that you wish, which randomly determines the effect.
(01-17) User loses physical control of weapon. A creature or object in range other than the intended target (not including the user himself) is struck by the weapon for half damage.
(17-33) (melee and thrown weapons only) User's weapon may take damage or be broken at the moment of impact. The intended target (which can be an object) gains a free sunder attack against it. If the user adds his Strength modifier to the weapon's damage, this is added to the opponent's damage roll. The sunder does not count against the target's allotment of AOOs, and does not provoke an AOO. If the target has any enhancements to its ability to sunder, these still apply.
(34-50) (ammunition weapons only) The weapon fails under mechanical stress, dealing normal damage to itself plus the user's Strength modifier. In the case of a critical fumble, the weapon explodes violently in the user's hands, dealing its normal damage roll to him and is destroyed outright.
(51-67) User loses grip on the weapon in the midst of a swing or thrust. The intended target (which can be an object) gains a free disarm attack against it. The disarm does not count against the target's allotment of AOOs, and does not provoke an AOO. If the target has any enhancements to its ability to sunder, these still apply.
(68-83) Awkward posturing or overextension of a limb wrenches the character's body in the wrong way. The character takes 1d6 nonlethal damage and is sickened for 1d6 hours because of the distracting pain and overstressed muscles. If the character was already sickened from a previous fumble, an additional -2 penalty applies from the sickened condition and the character becomes stunned for 1 round.
(84-100) (slings, chains, lassos, nets, etc. only) User accidentally entangles himself with the weapon and must spend a move action remove the entangled condition from himself and the weapon.
If a generated fumble event is inappropriate to the situation, reroll.
Rule #6. A critical fumble threat is resolved in the same manner as a critical fumble and imposes the same kind of effects, but the weapon's critical hit multiplier applies to any die roll imposed by the critical fumble.
Rule #7. Fumble Immunities: at 6th level and every five levels thereafter (11th, 16th, 21st, etc.), a character gains immunity to a fumble event of his choosing. Thus, if the fumble event generated is appropriate to the situation but the user is immune, then nothing bad happens at all (other than the fact of automatically missing on the original attack). Creatures gain immunities to fumbles according to their HD in the same way.
