When I was in fifth grade or so, I spent the night at my friend Kristen's house on Halloween. It was a weekend, so we were allowed to stay up watching scary movies in the front room. The front room had been decorated as a small haunted house, with lots of motion-activated decorations that made noise and moved around. Because we didn't want to be bothered by all that, we took the batteries out of every one of them and set them aside, then went about watching our movies.
About fifteen minutes later, all of the motion-activated decorations started moving, lighting up, and making their noises, and they wouldn't shut up. No batteries. No electrical power.
The overhead lights went out, too--though no one had touched the switch. I can explain away the lights flicking off (Kristen's house was old and probably had faulty electrics), but I'm still baffled by the moving, laughing, lightup Halloween decorations.
There was always something wrong with Kristen's house anyway. I don't know what it was, but there was always a heavy, "dark" kind of feeling around it, and you always felt like you were being watched by something not-nice. To this day, I don't like walking past that house in the broad daylight... and I'm twenty-two years old now.
It's not as creepy as some other stories, but it's true.
Another time, while camping in the desert, I heard something insistently pawing at and shaking my tent for quite some time before going away. It was probably a coyote or something like that, but it was still pretty goddamn creepy.