Not exactly an old famous mystery but still gives me the creeps. It was an old bed. My girlfriend bought it at a rundown furniture shop for \$100 and they drove it over in a minivan later that day. Some time shortly after she bought it, it started happening. I don't know how else to explain it except that when you were lying in the bed, and it was totally dark, there would be a scratching at one end, and the sheet would start to pull toward it. The bed and boxspring were on the floor so there wasn't an "under the bed" for anything to be hiding in or a frame to be catching the sheet. It felt just like when a cat is using the the corner of a bed as a scratching post. Like there were small snapping sounds when the sheets and mattress fabric were released from whatever was pulling on them. Very audible, and you could feel it. We hadn't mentioned it with anyone yet when a friend went to take in a nap in the gf's room then returned 15 minutes later saying, "there's something in your room and its scratching at the bed". We tore that room apart multiple times looking for a mouse or an incredibly strong bug or anything. We searched the mattress and looked under the piece of fabric that covers the box springs. Her room was small, we never found anything. After moving it happened once again. She called and had the store switch out her mattress because it was "broken." They did. Scratching/pulling stopped. And those terrifying nights of pulling at the sheets that were beging tugged off the bed remains a mystery to us all.