Critical failures are some of my favorite parts of Dungeons & Dragons as a Dungeon Master. I don’t particularly enjoy my players failing at something because I generally want them to succeed, but it certainly opens the moment for some interesting improvisation on my part. A healthy dose of random interjection keeps even the mundane parts of a campaign from growing stale.
I’ve introduced new enemies, added a whole layer of complexity to my world, and even killed someone else instead of the person who just rolled three 1’s in a row. People really ought to be more careful when they’re shooting into melee combat, really. They also need to stop accidentally summoning Outsiders to the material plane, thereby ushering in the eventual collapse of the universe because Outsiders are pure entropy and cannot be killed because entropy can’t be killed without breaking every law of the universe. And then you have bigger issues than entropy.
Aside from attack rolls, there are a few other critical fails that can be a lot of fun. Catching something or throwing something is a stat check using dexterity. If a player rolls a critical fail on a toss or a catch, it can be a lot of fun to describe what got broken by the fumbled throw. My personal favorite strength check failure was the giant, manly barbarian getting a splinter from the door he was trying to break down and being unable to do anything until he got it removed. A close second was the drinking contest. The Dwarf was trying to bond with the half-goliath barkeeper and decided drinking copious amounts of alcohol was the best bet. The dwarf lost, of course, but the fun was in describing how he got blindingly drunk and accidentally drank the barwoman’s dishwater. He burped bubbles for forty-eight hours because he didn’t even fish the bar of soap out of it first.
For saving throw’s, the fails are often a little more catastrophic. Just last night, one of my players turned into a water-breathing creature so he could avoid drowning in the swamp (a crocodile had tried to drown him and failed). Since it was a bunch of still, disgusting water that he spent a while swimming around in without doing anything about his open wounds, I had rolled a secret save versus disease, just to see what would happen. He rolled a 1 and thus caught an ingested disease because he kept accidentally swallowing swamp water while trying to breathe it. Good times. Waking up blind is always a great way to start the day.
In less extreme circumstances, critical failures just make for great flavor. Have someone critically fail their save versus a magical attack like a fireball? Throw in a comedic moment where they miscalculate and take cover behind something that’s just going to make the explosion worse, like a source of tinder or something easily flammable. Crit failing their Reflex save to avoid a trap? Have them dive the wrong way or have them just leap straight up in the air. Crit failing their Will save to see through illusion? Have them enthusiastically participate in the illusion. The possibilities are endless if you’re quick on your feet.
Past experience has taught me that there’s an important line to walk as a DM between throwing in extra penalties for critical failures and just adding flavor. If the moment is super tense and everything rides on this moment, be wary of adding flavor. If everyone is caught up, they likely have their own mental images of what is going on, so you want generic details that will meld with whatever they’re seeing. Penalties make this easier as you’re adding a new aspect to the image rather than changing something existing, and you can always add flavor on top of a penalty. If someone just failed something very routine, penalties can cause the session to drag, so extra flavor is usually the way to go unless you have something important hinging on this routine task.
The great thing about being a DM is realizing that all rules are situational and that you are the ultimate arbiter of what is right when you’re running a session. Figure out how you like to use critical fails and hope you get enough opportunities to put them to use. All that really matters at the end of the day is that everyone is having fun, whatever form that takes.