Dunno how popular this conspiracy theory is, but here goes...
Paul McCartney supposedly died in a car accident in 1966, and the remaining Beatles, particularly John Lennon, plus the band's management, felt it was best to hide this from their millions of adoring fans. They replaced the original Paul McCartney with an imposter: Billy Campbell, the winner of a McCartney look-alike competition, and paid off any policemen, paramedics, etc., that had witnessed the wrecked car and/or McCartney's dead body.
Opinions?
PS: I don't actually buy into this conspiracy theory, I just find it very interesting.