Event Category hidden on event add form

Hi guys:

This is very strange: I have 1 event category (and the child categories attached to that parent) that does not show in the back end administrative “add new post” nor in the front facing “add event” form. HOWEVER, it DOES show in Quick Edit for the post after it is published and I can edit from there. All the rest of the categories show just fine. It’s like this one category is hidden! What would cause that and how do I fix it?

Not urgent – I can still get to the category through quick edit – but it’s one of those things I’d like to clear up.

Thanks!