Fumbles happen the same way as criticals, just in reverse; roll a natural 1 on an attack roll, then roll a d6 for the table. There is one exception to this rule though. If rolling a 1 and adding all your modifiers would still result in a hit, you don't fumble, but you still miss.
1-2: Drop Weapon - Your weapon drops to your feet. You can pick it up on your next turn.
3-4: Break Weapon - You have to roll a die based on your weapon quality (thanks to Alexis; and an assist from the Jovial Priest just for the modified quality table he has) :
- Poor - d6
- Ordinary/Standard - d8
- Well Made/Quality - d10
- Master Crafted - d12
- Magic - d20
If you roll a 1 on this die, your weapon breaks.
5-6: Leave an Opening - You leave an opening and your opponent takes advantage and gets a free attack on you.
This should work and be fine. Also, easy enough to remember during play, much like the critical tables.
I haven't started using this yet. I've been just using the "Fumble = Target gets free attack on you". It's fine. It's quick too.