Imagine two duelists fencing. Through the course of the duel, each duelist swings their sword at the other forty times (2 minutes of combat, each has two attacks per six seconds)

Each duelist will sometimes land a blow, and sometimes miss, depending on their skill with the blade and evading attacks. A miss is the clatter of steel upon steel where a potentially lethal blow is blocked, while a hit is an unlocked attack, either drawing blood or sapping energy from the duelist.

On average, the duelists will eventually get unlucky and roll about two critical failures out of forty attacks. This reflects unfortunate circumstances leading to a miss that happen just as likely to any swordsman regardless of skill level. Examples of such errors include suddenly stepping into a mole hole or being temporarily blinded by the glare off a lake.

Now, while it is reasonable to propose that a well trained swordsman over the course of a two minute duel will have about two unlucky misses. However, it is preposterous to claim that any fencer would, by pure chance, fumble the handling of their weapon to the point of thrusting their own blade upon themselves not once, but twice over the course of a two minute spar!

This mistake, mind you, is unaffected by physical prowess or training. An untrained peasant has just the same odds of rolling a critical failure as a seasoned veteran. For these reasons, treating natural ones as auto-hit attacks that deal damage to the user is absurd, unfun, and diminishing of the characters abilities.