my older gm had a thing for natural 1 on attack dice rolls.she ruled that each is a fumble that result a check on a fumble chart she made. while this was fun and all. after the ranger of the group kept breaking his weapons for the 4th time in a raw,an idea came to me.
basically it is that the more attacks one have per round the higher the chance for him to roll a fumble. that seems unfair for the fighting classes and their high BaB rate.(at some levels it turned out that since only a 1 will indicate that the fighter miss. so actually ether he hit or fumble.never a simple miss chance) so when i started to game i made the following rule,as well as a new fumble chart.

1st: when ever a natural one is rolled we check to find out if it is just a miss or a fumble . the check is as followed :
the character's ( or monster,they get this too) BaB X 5% for a mere miss.so a level one fighter rolling a natural 1 will have a 5% that it is just a miss and not a fumble ,while a level 20 will fumble only if 100 is rolled.
this represent the fighting skill that comes with experience that separate the master from the skilled.

then if a fumble is affirmative i use the following fumble chart:

main fumble chart. roll 1d6:
1- a mere miss, no other ill effect
2- loss of balance :trigger an attack of opportunity .need to spend next move action to regain balance
3- twisted ankle (or any such organ),trigger an attack of opportunity and get a -2 condition penalty to attack and AC. last until magically healed or treated with a heal check at dc of 10.
4- fumble. roll a 1d6 on the secondary chart
5- bad fumble. roll a 1d4 +2 on secondary chart ( no chance for the 1st or 2nd fumbles)
6 -great fumble. roll a 1d4+2 on secondary chart and a 1d6 on this chart.combine effects.

secondary chart.roll as instructed before:
1- fell down.the character\monster is now prone
2- weapon drop .the weapon used for the attack is now prone (if unarmed then the result is as 1- fell down)
3- backfire. roll damage on the fumbled attacker using self damage mode.(DM can decide if full strength bonus apply)
4- critical backfire. as above only roll critical damage instead.
5- misfire. random hit a different target in range.if non exist treat as backfire
6- weapon break. find a way to repair it ( some gm might chose to damage the weapon per it's damage mode,so not every 1 shell break)