Welcome back to Scary Things! I'm keeping it a little mild for the next few weeks, as there's a tentative plan to have a fun creepy-fest in GT during October, so I'm going to save the really scary stuff for then.
Tonight I want you to tell us about something that was startling and frightening, but only lasted a moment, and that never had a good explanation afterward. A real-life jump-scare, if you will. I'll get the ball rolling:
I was living with my best friend about nine years ago; I actually rented a room from him in the house he owned, and most of the furniture was his. It was a nice enough house with plenty of room, two adorable cats, and no ghosts that we knew of. The living room was largely empty, except for four seven-foot-tall fully-laden bookshelves, a grandfather clock, and a chair (this is important to know).
One evening, we were watching a movie in the TV room, which was off the living room at the back of the house. Due to the angle of the door, we couldn't see into the living room, even though it was only separated from us by a pretty flimsy wall. Suddenly, we heard a house-shaking crash from the next room, that very clearly sounded like at least two of these giant bookshelves toppling over - there was literally nothing else in the house, except for maybe the house itself, that was big enough or close enough to produce that crash. We jumped up and ran into the other room, thinking that maybe the cats had somehow toppled a shelf and started a chain reaction, and we were frankly worried for the cats' safety.
The room was fine, the shelves were upright, and the cats were totally undisturbed aside from apparently wondering why we were so worked up. Nothing in that room was out of place. Nothing in the house was out of place.