Tales of a GM is running the 12 Days of Dicember, a project dedicated dice in all their randomized glory. The question for the fifth day of Dicember is “What is your rule for when dice roll off the table?”
Easy answer: re-roll it on the table. Doesn’t matter if it’s a critical hit or a critical fumble; if it hits the ground, you re-roll it.
A related rule is that a die must end its roll flat; it can’t be a “leaner” (meaning the top surface of the die can’t be at an angle). Dice that end up on a seam or crack may need to be re-rolled. To test, place another dice on top of the uneven die; if it slides off the die, the original die needs to be re-rolled.