I was maybe about 8 years old, home alone when my dad came home from work. He'd already yelled at me to stop watching TV, but I turned it back on when the coast was clear and he went to his bedroom (other end of the house). I was trying to watch cartoons on the sly in the living room, when I saw his reflection from the kitchen in our sliding glass door. I quickly snapped the TV off and then turned back around, only to see nothing. It was bizarre, because I absolutely had seen the reflection of a dark, tall outline of a figure walking through our kitchen. I was freaked out and confused, and went to go look for my dad. He was still in his master bathroom, washing his face, and said he'd been back there the whole time.
Gah, I haven't thought about this in such detail in a long time, so I hope I don't have nightmares tonight. It scared the living shit out of me, and still does.
(I hope my description of the layout isn't confusing - basically the sliding glass door was in front of and between me and the kitchen, so from my view I could see the whole reflection of the kitchen and shadow of someone walking around). (courtesy of Jezebel)