My players tend to roll a lot of 1's. I know everyone says that, but seriously, a couple of these guys would trip and cut their own heads off if it were an option (and it has been). So when players roll a '1' in my game I've tried livening up the event through various means.
First I tried making rolling a '1' always result in dropping your weapon and falling prone. This got old and boring fast, as sometimes there just wasn't a good reason a character would drop their weapon and then fall over. Throwing a dart at an orc? *Slip! Plop!* The first time that's funny. The fifth time its just dumb. Plus it quickly felt like it was being forced upon the players, rather than a ridiculous event. Every time they rolled a 1 they fell down and suffered the -4 to AC, etc. It just became a mean penalty rather than a hilariously spontaneous event.
Second I tried those Piazo Critical Fumbles cards. They are a deck of random little cards that each have effects listed for the standard kinds of attacks on each card, so no matter whether you were trying to shoot a bow (Ranged) or swing a greataxe (Melee) there is an entry on the card for you. If you roll a '1' (or fumble equivalent), draw a card, and suffer the effects. The problem with them is that they are heavily 3.5e/D&D-centric, and I'm a renaissance gamer who likes to run many types of games. So the entries saying "You take 1d6 ongoing bleed damage unless Fort 15 save is made" didn't always have obvious or quick ways to implement in non-D&D games. They aren't great for Hollow Earth or Call of Cthulhu, for example. Also since they couldn't be situational to what was going on, they kind of pulled you out of the combat rather than made it more fun.
I realized I wanted a critical fumble to have both random effects but also be conditional to the situation (i.e. on muddy ground make it more likely to slip, etc), as well as make it possible for a 1 to just be a regular failed attack. This meant that I couldn't count on a pre-written deck of cards or a percentile chart list of pre-written calamities to roll on (as much as I love percentile charts). I also needed it to be quick, so we don't take up a bunch of time looking up a chart of effect, finding that other d10 to roll on the percentile chart, and then reading a bunch of text.
So the requirements for a Critical Fumble system are:
1 - Random
2 - Situational
3 - Quick
Soon I discovered my preferred system, which I will share with you now:
THE D6 OF PAIN!
Here's how it works: When a player rolls a '1', we all laugh and then stop everything for a couple seconds while they pick up a D6 and I list the negative effects that the player will suffer on a 1-3. On a 4-6 it is always considered a regular failed attack/test. But on a 1, 2, or 3 something bad happens to the player. Its best to list these three things very quickly before they roll the die, so if the relevant number comes up its not perceived as punishing the player willy-nilly, because it was a random effect. I like to make a '1' on a d6 be the worst effect, followed by a slightly less-bad effect on a 2, and the "least-bad" effect on a 3. In all cases the three effects are related, and are best when the bad thing can also happen to another player.
The Barbarian fights next to the flowing stream in the wooded glade and swings his greatsword at the ambushing Orc. The player rolls a '1' (on d20). He then sighs, picks up a D6, and waits to roll it as the DM quickly comes up with three things that could happen based on where he is/what he is doing.
On a 1 (on D6) - "The Barbarian accidentally throws his sword into the waist-deep stream about fifteen feet away, and it 'dunks' into it. He'll need to draw another weapon or go wading in to retrieve it from the stream bottom (assuming he can locate it quickly) next round."
On a 2 - "The Barbarian loses the grip of his sword and it sticks in the mud a few feet away. He'll have to pick it up next turn if he wants to keep fighting with it."
On a 3 - "The Barbarian accidentally slips on the muddy bank, and falls to one knee (still holding his sword). Minor penalty to AC on the next round."
On a 4-6 - Its always just a regular failed attack (no penalty). ...Who's next this round?
That's kind of a boring example but gets the point across. It's also good when one player's fumble affects another player's character in some way. While you'd think it would create animosity at the table, it in fact more often builds comrodery, because the affected player knows the son-of-a-bitch who made such an effect possible is the DM, not the player, and they all team up in their anger against the DM (which is what you want, right?). But really they learn to fear that dreaded D6 roll (and secretly get excited when someone else has to roll it).
The Elf Archer is helping his friends who are battling the goblin mercenaries in the urban ruin of Mordheim. He fires his longbow into the melee where his companions are fighting desperately. He rolls a '1'. He picks up a D6 and awaits the list of fate:
On a 1 (on D6) - You hit a small piece of unstable wierdstone that one of the Goblin mercenaries was carrying on his belt, and it explodes into blinding powder right in the face of the Fighter. The Fighter needs to make a Save vs. Magic or be blinded for the next 1d4 rounds.
On a 2 - You hit the Dwarf Berserker in the back. He takes damage and you can probably expect your name to be put into his personal book of Grudges.
On a 3 - You hit one of your companion's shoulder pauldrons, causing him to mis-step for a moment. You do no damage, but the Goblin mercenary he was fighting uses the moment to slip past him and come rushing at you. You'll be in melee next round.
On a 4-6 - Regular miss.
So with a D6 used as the critical fumble penalty-determiner, and half the roll results being nothing, you only have to quickly list three things that could happen, of descending penalty, based on what's going on in the moment. Once the players catch on, you can actually list what happens on a '1' (the worst result), and then go around the table and ask another player what could happen on a '2,' and they could work off of what you listed for the 1 penalty, and then another player what could happen on a '3.' The importance here is that its quick, as if the player just rolls a 4-6 then it won't come to pass, so don't waste a lot of time on it. The best case scenario is that you list three bad things, they roll a 5, and realize how they barely dodged a bullet!