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.
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.