Beetlejuice - They Aren't Supposed to Have Reflections
Ghost's aren't supposed to see their reflections, right? Well, whenever the two ghosts look in the mirror, they don't see their reflections, but whenever they are in front of windows, you can see their reflections just fine.
Yeah, Agnarth, In the beginning of the movie they actually make a point that they can't see themselves in the mirror, that's how they find out they're dead.