In a nutshell, this is how we do fumbles to avoid the '5% of the time you fumble' rule.
We use exploding dice, so if you roll a 1, you roll again and subtract it from your bonus. If you subsequently roll a 20 you roll and subtract again. If your total adjusted roll is 0 or less you fumble; so (for example), I make an attack at +13 total but I roll a 1. Crap. I roll again (the exploding die), and if I roll a 10 my overall roll is a 3- no fumble. On the other hand, if I roll a 16 when I roll to explode, my total is a -3, which is a fumble. Depending on how low your overall roll is, your fumble severity varies (to get the worst severity, you must roll worse than -20).
One thing that has been pointed out as a problem with my system is that iterative attacks are more likely to fumble, so a character who is a really good fighter is also more likely to fumble. I'm not quite sure how to address it, and I agree it's problematic; it's a step back towards the 5% fumble rule.
Nonetheless, my group finds fumbles fun (I think) and overall agrees that having them is more realistic than not having them.