In the first episode of He-Man (1983), Skeletor approaches Castle Grayskull with his cronies, and says:
Without He-Man to defend it, Castle Grayskull will be ours!
Then the door is forced open, with the help of Tri-Klops, revealing He-Man behind the door. Tri-Klops has the ability to see through objects. Why didn't he see He-Man?