Nat 20's SHOULD always succeed, because if the player can't succeed no matter what they roll, the option to roll shouldn't be presented to them. Don't let them do stupid shit like the wizard with 6 strength leaping 100 feet in the air or getting shot by crossbow bolts 20 times and taking no damage, because that is an impossible feat, no matter what they roll.
How Far Should A Nat 20 Take You?
Adrian Jones
Evan Bell
>getting shot by crossbow bolts 20 times and taking no damage
Note, though, that if he *does* somehow out-AC 20 crossbow attacks, that's just poor marksmanship. It's generally a bad idea to take away combat rolls unless it's impossible to miss.