Critical Failures & Fumbling Rules

Chances & Values

A critical failure is, depending on the game system you use and whether they use them at all, a particular d20 (20-sided dice) roll that lands on a 1. This usually means whatever you're attempting, whether an attack, persuasion or something else, fails no matter what skills and bonuses you may be able to add to that 1. This means 1 in 20 actions fail regardless of the circumstances, assuming you're using critical failures. This in turn could mean that somebody who rolls a 1, but has +7 in bonuses to add, would essentially roll worse than anybody who rolled a 7 or lower. This usually doesn't matter as characters rarely have +7 to add to their actions until later in the game, and that rolls that low generally are a failure anyway. But when critical failures have a big effect, the difference between a 2+0, a 2-3 and a 1+7 can suddenly become far more impactful, which is something to keep in mind.
The same can be said for a natural 20+0 and a 19+5, but critical successes are generally more accepted. That doesn't mean you can't bring this up in your game as well though, plenty of people alter whatever existing rules they play with to suit their group's play-style best.

Another thing to keep in mind is the fact that, again depending on the game system you use, some classes rely on attack rolls far more than others. Any weapon attack is usually based on such an attack roll, but there are plenty of magical attacks that rely on a saving throw from whoever is targeted, so these attacks negate the chance of a critical failure on the side of the caster. Why does this matter? Because a character who can only attack with a weapon will have far more chances to suffer whatever consequences are attached to a critical failure than a character who primarily uses attacks that require a saving throw to be made by the opponent.

What they can add

Even with the before mentioned elements in mind, critical failures can still add a lot to a game. Most people use them for the sake of hilarity. Any action can quickly get way out of hand on a natural 1, which, combined with a fumble chart for example, can mean dropped weapons, self-injury, botched negotiations, accidental marriages, and a whole range of stories shared on the internet. Spectacle, madness and absurdity are usually the main additions in groups who use critical failures as they're perfectly suited for humorous, Monty Python-esque adventures. But there are more ways to take critical failures.

Darker or high-risk stories could work well with critical failures, either with or without ignoring critical successes. There are other ways to go for high-risk of course, like using stronger monsters and lowering supplies, but using critical failures adds an element of dread to the players, as critical failure is possible with (almost) every roll. Consider it as the difference between having a chance of a critical success against a main villain, which often makes for a heroic moment, versus a chance to have a critical failure the villain takes advantage of or causes the character to fail in the eyes of their peers. But let's not delve into how to make critical failures work just yet.

Beyond story elements, critical failures can also add a balance to critical successes. Many systems already have something in place for this, but there's nothing stopping you from adding to it. Critical successes can lead to challenges that are overcome too easily, so in some campaigns it can make sense to have a critical failure to make challenges more difficult as well.

What they can take away

Critical failures can take away a lot. The competence of the characters is an obvious one, because while a critical failure might lead to something as seemingly innocent as a dropped weapon, a competent sword fighter wouldn't lose their weapon 1 in 20 attacks. Of course a critical failure doesn't have to mean a weapon is dropped every time, but it is worth keeping in mind what the effects of failures this frequently means. This also means that besides the competence of the characters, critical failures can also take away some form of realism. This is very relative of course, because we're playing in a fantasy world to begin with.
Another, related element critical failures can take away is game flow. If something grand, strange or hilarious needs to happen on a critical role, it can disrupt the flow of the story, the game, and thus the enjoyment of the players. The reverse can be true as well of course, it depends entirely on your group. Some enjoy a social encounter that could lead down a rabbit hole of insanity with a few critical failures, others prefer the social encounter to simply take a different, but still relatively normal turn.

I touched on fairness earlier on, so I won't delve too deep into it, but it is worth to plan ahead if balance is of importance. There are plenty of campaigns that could work great with some unfairness as well though, like a Dark Souls-esque campaign where death lurks around every corner. Critical failures taking away elements of the game isn't necessarily a bad thing, simply a result of a mechanic like any other.

Making critical failures work

There are plenty of ways to make critical failures work, but some will work better than others for your specific group. It's always best to talk with your group to figure out some of the details in order to avoid nasty surprises down the road. Many players don't think about the mechanical implications for various classes, for example, so it's worth pointing those out to them. If both the mage and the barbarian are fine with the barbarian have a higher chance of encountering critical failures, then by all means use them. If not, try one of the solutions below.

Only on first attacks

A common way to negate some of the imbalance of roll differences between classes is to only use critical failures on the first attack made by a character. So if a ninja or barbarian are able to make 3 attacks per turn, they can only have a critical failure on their first attack. Any natural 1s on the second or third attack are simply counted as an automatic miss.
This doesn't remove the imbalance entirely, a wizard or grenadier can still just use an ability that requires opponents to dodge or make some form of defensive roll, but at least it'll be a difference of 1 to 0 chances instead of 3 to 0 chances between the classes.

Failures give advantages

A critical failure doesn't have to lead to a dropped weapon or self injury, it can simply lead to an advantageous moment for the opponent. So whenever a natural 1 is rolled (or only on the first attack), it could give the opponent advantage on their first attack roll if they make it against the character with the critical failure. It'd be like leaving an opening in your defenses, which can be exploited, but isn't always. A roll with advantage doesn't always mean a successful one after all. The other characters could try to stop a particularly powerful opponent from being able to use this moment of advantage, by freezing or blocking them for example, if they are able to act before the opponent's turn.
The added bonus of this method is that it increases the sense of dread on a natural 1 roll. The actual consequences of the failure won't be acted out until the opponent's turn, so there's a period of "I'm screwed" against some enemies who can exploit advantages fully, while other characters may desperately be trying to save the other character. It makes the battle more dynamic and responsive too, rather than simply having a weapon be dropped or self injury leading to random added damage. That's not to say a dropped weapon doesn't add dread too, but I personally feel like they take away from moments of deliberate attempts to disarm an opponent.

An extra roll

Another way to add big failures without having them happen 1 in 20 times is to have players roll another d20 to see if or how badly they screw up. You could tie the roll to their levels, so any roll equal to or lower than their level means they simply miss and anything above means a bigger failure. This way failures happen less and less often as the characters level up.
You could use any bonuses they may have in the skill they used as well. This way failures are less likely in skills the characters are trained in and more likely in those they're useless at to begin with. So you could end up with something like the following:
- A level 5 medic tries to heal somebody, but rolled a natural 1. They then roll again to see how badly they messed up. They roll a 9, that's above their level so it would mean they messed up big time, but because they have a +2 to wisdom and are +3 proficient in medicine, they can add those to their level of 5 giving a total of 10. This means they simply failed to heal a character, but didn't mess up in a way that makes things worse.
- A level 10 barbarian tries to subdue a bear by calmly talking to it. They roll a natural 1 and have to roll another d20 to see how badly they messed up. They roll a 9, but have a wisdom modifier of -2 and no proficiency in animal handling, so end up with a total of 8 instead, meaning they messed up big time and the bear is now infuriated.

There's plenty of ways to take this and you can make it as complex as you and your group like. Perhaps not being proficient gives an automatic -2 penalty. Maybe rolling a natural 20 on this extra roll means the mistake was overcome at the last second and instead of the original 1 counting as an automatic miss, proficiencies and other bonuses can be added to see if that 1+7 I mentioned at the start could be worth something after all. Generally this won't do anything as even an 8 isn't that great of a roll, but it can keep the specialness of a natural 20 going to some degree.

Flavored descriptions

An easier method is to simply describe failure with added flavor. This way you can dose the severity of failures without the need of rolls, and make them easily applicable to specific situations. With a good sense of humor you can make them as funny as you wish too.
This method allows for a wide range of consequences, but can make for character development moments or other story elements. The shame of failure at crucial moments can weigh heavy upon characters for example, while failure during less crucial moments can lead to moments of humorous upstaging between characters or awkward moments of attempted intimidation, seduction or other social moments.

Descriptive story telling is usually a building stone of role playing games, so implementing this is rarely difficult. It all depends on how creative you are, how creative you want to be with your campaign, and on what the group is looking for. In the end everything boils down to having fun with a bunch of people, so find out how to best do this, adapt critical failures to suit this, and you'll likely be good to go no matter the methods you use.