The guy who taught us to play D&D 4e, after playing in my HackMaster 4e game for a while, decided to institue a fumble table, but had us confirm our fumbles before applying the potentially very nasty effects. Confirmation involved rolling the attack again: if you hit, it's a normal miss; if you miss again, it's a fumble and you now roll on the fumble table. It was a blast – especially when the rogue, who can normally roll high enough to hit, even on a fumble, managed to confirm a fumble and ended up stabbing the mage who had once again rushed into melee like an idiot. Good times. 8o) As of yet we haven't seen every result on the table so we players don't know its exact contents so I doubt he'd be willing to post it here.
My favorite crit/fumble system is in HackMaster Basic. One thing you must know is that HM5e uses opposed rolls almost exclusively. In combat both players roll a d20 and add the appropriate modifier; tie goes to the defender. That means both players can crit or fumble.
If the attacker crits (and rolls higher; nat 20 auto-hits regardless) damage is doubled as is common, but if the defender crits he gets to make a counter attack. We've seen attacks go back and forth several times in the same second* – parry, reposte, parry, reposte, ... – until a solid attack finally got through. On a Near Perfect Defense (defender wins with a nat 19), you get to make a free attack with a dagger or smaller weapon, or headbutt, knee to the groin, or however else you want to flavor it, for weapon damage or 2(d4p-2) ("p" means the die penetrates so you can roll more than the max; I hit for 30+ damage with 4d4 once; the hit player was unhappy *G*). If both players roll nat 20s, the higher total wins.
* Combat in HM5e is in pseudo-real time instead of rounds. Everyone can act on every second, but attacks are limited by weapon speed as long as you are engaged.
If either player rolls a nat 1 and has the lowest result, it's a fumble and the other player gets to make a free attack on the next second.
The requirement that the nat 20 must also be the highest result to be a crit, and nat 1 must be the lowest to be a fumble, makes crits and fumbles occur slightly less than the standard 5% of the time, and I like that a lot better.
The newly released HackMaster Player's Handbook 5e adds a proper fumble table and splices it into Basic's system I just described, but our group voted unanimously to keep going with the simpler yet highly fun and exciting Basic system.
After I typed that, and while I waited for our Internet to come back up (it goes out every time the wind blows the lines; doesn't matter if we have Cable or DSL), I grabbed our delicious new PHB (there is nothing like the smell of leather) to see how its fumble table compared to the previewed one in an issue of Knights of the Dinner Table, and made a surprising discovery: it doesn't have one. So that paragaph should read:
An issue of Knights of the Dinner Table added a proper fumble table and spliced it into Basic's system I just described, but our group voted unanimously to keep going with the simpler yet highly fun and exciting Basic system.