The mechanics of the Critical Fumble, i.e., rolling a ‘1’ on a d20, and the Critical Hit, i.e., rolling a natural ‘20’ on a d20, have been part of D&D for many years but never were cannon rules that I remember. In 3.5 Manaverse, so far i think the natural 1 and natural 20 is only being applied to the following

Critical Success & Failures apply to:

Attacks: Melee Attacks, Ranged Attacks, Spell Attacks Saving Throws: Relex, Fortitude, Will

They DO NOT apply to:

Initiative Skill Checks Caster Level Checks (spell resistance as an example) Magic Resistance

Caution: Subject to change based upon the whims or sobriety of the DM.