Well, maybe it's better to say The Blob was based on a "true" story. No one got sucked into a garbage disposal like in the horror movie classic, but reportedly one policeman's hands got sticky and that's equally horrifying.
Both the 1958 and 1988 Blob movies are "based" on a real incident that happened in Philadelphia in 1950. On September 26th, two officers on patrol saw what appeared to be a parachute float down from the sky. They drove to the landing site to find a... something.
Various accounts give it different qualities. It seemed to be about six feet in diameter, purple, filled with strange crystals, and giving off a mist. Naturally, one of the police officers put his hand right in it. It left a "odorless sticky residue" on his hand, but fortunately didn't eat him and his fellow officers, giving the terrified populace no choice but to freeze it and fly it to the arctic. Instead, it seemed to sense that it wasn't wanted, and sadly dissolved into nothing, supposedly leaving the grass underneath it unbent. During the 25 minutes it spent on Earth, two more police officers were called, as well as the FBI, although the agents didn't arrive in time to witness anything.
The story was reported locally, then picked up in the national press as a sort of joke. No one knew what to make of it, so no one treated it seriously. The FBI asked that the Air Force look into it. The Air Force declined. What did the officers actually see? No one knows.
[Via The Blob]