I would have said Skill Focus, but it is often a prerequisite for a Prestige Class. So I don't wonder why, say, a wizard with a rank of 10 in Knowledge (Arcana) and Knowledge (The Planes), and more than seven known Divination spells, take Skill Focus on a Knowledge skill.
Same thing for Toughness, a Dwarf fighter that take the Toughness feat, I don't wonder why.
So, I answered Armor Proficiency. As far as I know, when a class don't give you an armor proficiency, the armor you can't use would nonetheless impair your class abilities or skills.