He doesn't appear in "Errand of Mercy."That was not the only episode that Bones was not in. There were a couple of other episodes in the 1st season he was not in. I should know I have both season 1 & 2 on blu ray and have watched every episode on both seasons. In season 2 Bones is in every episode, but in the 1st season there are at least 3 or 4 episodes he was not in. One of the episodes from season 1 besides What Are Little Girls Made Of Bones didn't apear was Where No Man Has Gone Before and couple others I cant' recall at this moment. He also didnt apear in The Cage, but Kirk was not in that either.In What Are Little Girls Made Of?, Bones didn't even appear!
Obviously, I'm not considering The Cage, or Where No Man Has Gone Before, since technically, they are pilots, and not regular episodes. As for Bones not appearing in Errand of Mercy, which honestly, I don't remember, that bit of trivia just completely blows my theory straight to Hell!