Ah, got one. When I was quite small (don't quite remember, going to say seven or eight) my mom and dad tucked me in for bed, then went out for an evening walk. My bedroom door was left open, and the hall light was left on. Just moments after I heard the front door close behind my parents, the hall light turned off, then on again, then off, then on again, then off, then on again. This went on for several seconds, then the light remained on. I could have wrote it off as wonky wiring, or something with the power, that would make perfect sense. But the thing that made my young self's blood turn cold, hair stand on end, and freeze entirely in place, unable to move anymore than other than pulling the covers up to my neck, was that each on and off of the light was accompanied by the sound of the click of the light switch.
The next morning my young skeptical mind came up with the theory that someone had slipped into the house after seeing my parents leave, and flicked the light switch (which was very near the front door) on and off looking for a reaction so as to determine if the house was abandoned or not, and my choking out the words "Mom? dad?" was exactly what this fellow needed to tell him that the place wasn't deserted and that he needed to find another place to burglarize.
Of course, there was also my older sister whose room was down stairs to consider. She was just the mechievious sort who'd try to do something like that to freak me out. But for some reason my young mind, despite the fact that it was always suspicious of her about everything, never considered the possibility of it being her. Maybe it is because I didn't hear her come up the steps, or because she was sleeping over at a friend's that night. I don't remember, but at the time I was certain that it couldn't have been her for some reason.