[Myrroddin]

Protected usually means we can use it under certain restrictions. For example, a protected API may not work correctly, if at all, in combat.

In WoW-speak, calling Show and Hide on a frame is alright in combat, provided you created the frame beforehand. However, if you tried creating the frame in combat and then called Show on it, you would get Interface Action Failed errors, and nothing would happen visibly. Same goes with changing a frame's attributes. Those have to be defined out of combat, and cannot be changed during.

Without me actually looking at the API list (being lazy here) I can't be more specific. But most of the time, calling protected APIs in combat will not work.