Tips for using fumbles/critical failures in your tabletop game:
1. Don’t. Critical failures are typically appropriate only for explicitly comedic games. This isn’t to say that funny things can’t happen in any game – certainly they can! – but by employing critical failures, you end up with a game where the rules themselves mandate a certain minimum level of slapstick bullshit, regardless of circumstance. It doesn’t hurt to consider whether that’s actually the kind of game you want to run.
(Note that comedy doesn’t just mean Looney Tunes. A dystopian bureaucracy milieu where people die horribly for making mistakes on paperwork is also a comedy game – for certain values of comedy! – so critical failures may be appropriate there. This guideline isn’t intended to restrict, but to encourage you to think about the rules in terms of what style of play you’re trying to foster.)
2. Respect player agency. A common error of novice GMs is to use critical failures as an excuse to hijack players’ characters and have them do things they’d never do voluntarily simply because it would be funny. An example I see frequently is “you fumbled your First Aid roll, so instead you stab your patient in the face with a knife”; that’s good for a cheap laugh, but unless the tone of your game is straight up Looney Tunes, it’s not a reasonable outcome of simply being very bad at first aid. Bad dice rolls represent errors in performance and judgement, not random demonic possession.
(Obviously narrative context is important here; “my character is possessed by a minor demon that forces her to do something pointlessly evil every time she rolls a fumble” and “my character is a literal space alien who often harms people unwittingly because she doesn’t understand how humans work” might both make the face-stabbing thing acceptable, because now it reasonably proceeds from established characterisation. Those are outliers, though.)
3. Keep the magnitude of fumbles commensurate with what was attempted. Inadvertently starting a war by critically failing a Subterfuge check to lie to the King may be reasonable; inadvertently starting a war by critically failing a Streetwise check to gather rumours in a tavern typically won’t be. Barring exceptional circumstances, players should have a reasonably good idea of what’s at stake whenever they pick up the dice, and disproportionately harsh critical failures make it impossible ever to know what’s at stake.
This also applies across classes of activities. If the typical result of fumbling an attack roll is “you pull a muscle” and the typical result of fumbling an Athletics roll is “you slip, break your neck and die instantly”, the message you’re sending to your players is that you’d prefer them to resolve their problems with fistfights rather than footraces in your game – which is a problem if that’s not the style of play you’d intended to encourage.
4. Don’t foreclose: redirect. A critical failure that simply blows the players’ plans out of the water and renders whatever they were trying to achieve impossible is bad not because it’s unfair, but because it’s boring. Blocking is poor improv, and it doesn’t become less bad just because the dice gave you an excuse. A good fumble simply creates complications that need to be addressed, or shuts down the players’ current approach while creating or highlighting a different route to the same goal.
(This is especially important to keep in mind when your players’ plans require more than one roll. Even if the odds of critically failing any one roll are very low, the likelihood that at least one of a long series of rolls will turn up a fumble is very high. If any one fumble renders the whole plan impossible, nothing will ever get done. Most tabletop RPGs are strongly informed by heist fiction, so take your cues from capers: disasters and opportunities are the same thing.)