Ultimately if 1 will not fail or 20 will not succeed, why are you even rolling? While there is no default automatic success/failure rule, it's a natural assumption that 1 and 20 are automatic fail and success based on the fact that the roll is pointless otherwise.
As someone has mentioned, "Pass" and "Fail" are not the only possible outcomes of any given roll. That's why there are numbers on the dice besides 1 and 20.
Also, the GM doesn't usually(and also shouldn't, with everything else they need to keep track of) memorize every aspect of all their players' character sheets - they don't necessarily KNOW if the check is impossible to pass or fail.
Ultimately if 1 will not fail or 20 will not succeed, why are you even rolling? While there is no default automatic success/failure rule, it's a natural assumption that 1 and 20 are automatic fail and success based on the fact that the roll is pointless otherwise.
Tiered DC. I don't know if 5th has that as an official rule or it's a common house rule.
Rolling to seduce a dragon? Nat 20: he/she laughs at you, you fail. Nat 2: does a 22 hit your AC?
It's mentioned somewhere in Chapter 8 of the DMG.
As someone has mentioned, "Pass" and "Fail" are not the only possible outcomes of any given roll. That's why there are numbers on the dice besides 1 and 20.
Also, the GM doesn't usually(and also shouldn't, with everything else they need to keep track of) memorize every aspect of all their players' character sheets - they don't necessarily KNOW if the check is impossible to pass or fail.