Harry Potter and The Paranoid Professor
The new Defense Against The Dark Arts Professor was Alastor "Mad Eye" Moody, whos false eye he could pop in and out, could swivel in all directions, and had x-ray vision.
Moody invited Harry over to his office to give him some extracurricular self-defense practice. "The most important words to remember in your head is CONSTANT VIGILANCE," Moody said as soon as he shut the door, his eye spinning sinisterly. "You never want to think you'r...e SAFE because you NEVER ARE. You never want to depend on a PERSON or a PLACE or an INSTITUTION for your safety. You never want to think you're ALONE and UNWATCHED. You ever heard, 'You watch your back and I'll watch mine?' Losing my meat eye was the best mistake I ever made." He went off for a while about the surveillance state, and then paced over to a small mirror on his desk, his eye spinning sinisterly.
"This is a foe glass. It shows me which people I shouldn't trust around here." Harry looked into it and could see lots of vaguely familiar shadows, which reminded him strongly of the Hogwarts staff members. "You SEE Harry, trusting someone is just a temporary decision, it doesn't really tell anything about the future. These foes in my foe glass? They're my friends as much as I'm theirs, until they decide not to be. I don't have to worry about it until I see the whites of their eyes," Moody said, grinning, his eye spinning sinisterly.
Mad-Eye Moody turned out to be a random other guy in disguise, and he betrayed Harry to Voldemort. The false eye, actually, turned out to secretly be yet another guy (on nobody's side), who wanted to keep an eye on things at Hogwarts for reasons of her own.