Sometimes misfortune plagues an attack: The attacker stumbles slightly, makes a poorly timed weapon swing, or otherwise completely misses the mark. No bonus can turn the attack into a hit. It simply misses.
Natural 1: When a creature makes an attack roll against a target and a 1 comes up on the d20 (this is called getting a natural 1), the power automatically misses the target. Bonuses and penalties don’t matter; the target is simply missed.
Published in Rules Compendium, page(s) 216.