Dimensionfold Wiki

When attempting any d20 success roll, you have a 5% chance of rolling a “1”. This is called a Critical Fail. On top of failing to perform the desired action, some other negative side effect will occur.

Not only do you fail to do whatever you were attempting, you also have to deal with other bad stuff. The GM will think up something interesting I'm sure. Maybe you drop your sword and it sticks in your foot. Maybe your fireball spell hits your friend instead of your opponent. Stuff like that.

see Optional Rules

(You also have a 5% chance of rolling a “20” which is a Critical Success)