I've always, well, I suppose I should say "usually", treated fumbles very light and free-form.
A natural 1 was an automatic miss and the consequences usually involve the losing of the next turn (the weapon is dropped, the attacker missed
so badly they are too off balance to recover for the following round, your weapon becomes stuck in the side of a nearby tree, whatever the surroundings will allow.)
But I have been in games, in the 1-2e days, that used absolutely
brutal tables. The DM of the longest running game I played in created her own percentile chart (100 options) with everything from "the weapon is dropped/lose a turn" to "your weapon breaks" (if it wasn't magical) or "you hit an ally for max damage" to "self-maiming" and even (a 00 roll) "self-decapitation".
Often times, these results were so far-fetched and ridiculous that enough griping by the players usually resulted in the DM altering the outcome. But sometimes not...the need to reattach limbs was fairly commonplace back then.
(In all -what we then thought of as- "fairness", a similar table was used for natural 20's that had equally brutal results...for the enemy. There's nothing quite like the round of cheers that go up when a PC cleaves the head from the BBEG.
I tried DMing with that table for a time (and tried some other tables over the years) and found it just too...disruptive to the story.
So I just make up the fumble results on a case-by-case basis, in some way that makes sense to the existing scenario. Bad things can happen, you might even hit your ally who is fighting the same foe next to you (esp. in close quarters)...but you're not going to miss so badly that your weapon flies 20' (or more) across the room to strike you nearest ally because a random dice roll says so. You will miss. You will, likely, lose your next turn to recover (either grabbing your weapon, drawing a new one or righting yourself, from tripping/being off balance)...but chances of cutting off your own head...mmmmmyeah, prolly not gonna happen.
So when I DM I generally try to keep the descriptive/narrative portion of the fumble to something that...I always get in trouble with this phrase, but...something that "makes sense." It can be a terrible outcome, but it should not break everyone's immersion.
Get creative, get cruel, get ridiculous (if that's the mood of the game you are playing) but have the description make sense.
Personally, I think working without a chart is more fun (for the players and DM) and challenging (for the DM) since it doesn't take away the "Oh S***! I rolled a 1!" factor. And as the DM, I don't have to come up "on the fly" with some way to explain that "You cut off your own leg...at the hip"!?! Which almost immediately leads one down the "farcical" road. I
do have to come up with some "realistic" or "reasonable" result that fits the existing scene.
Leaving the results up to the DM also will avoid the example provided where the dice dictated, over and over, that there were "helmet malfunctions". ..unless, as someone suggests, you want to write that into part of the story (which I think is great way to handle the above if I were using a table to define the results and had 3 identical rolls).
If it is just a bother to be narratively creative on the spot, then do away with the fumbles. Natural 1 is a miss or miss and lose the following turn. Period. But then, make sure the monsters/foes suffer the same (i.e. "can't critically fumble" per se).
There's my coppers.
--Steel Dragons