Good fumble rules

[SinsI]

Less egregious fumbles(and only for mundanes), and you get to choose if you take the fumble in exchange for an action point.

That one is not problem free either: 10th level warrior dons his armor, gets Fast Healing 1 and some DR, grabs two whips and picks a fight with some CR 1/3 creature that has natural armor 3 or greater. The fight lasts till that creature kills itself due to fumbles and the warrior ends up with a bunch of extra Action Points.

It might be better to attach Fumble rules to story appropriate moments and challenges, and not to arbitrary in-combat moments. I.e. failed fumbled Climb check for a cliff can make that whole cliff collapse...