Can I change my answer? I've just thought of another one that I think is even more ridiculous than the faked moon landings example I gave.
Beatle Paul McCartney supposedly died in the mid 60s and was replaced, at short notice, by a look-alike and sound-alike that could play guitar and bass left-handed and could write world-class hit songs in exactly the same style. The idea that Paul's early Liverpool fans (who actually knew him), or his friends and extended family could be fooled (or successfully sworn to secrecy) is beyond belief as is the idea that the new guy's friends and family could keep it secret too.
I would only have classed this as a rumour and not a conspiracy theory if it weren't for the fact that I recently discovered it's still going on (Youtube, of course) but now MI5 (The British Secret Service) are cited as the ones who arranged it all. (If anything, the establishment would have been glad to see the Beatles split up).
For anyone remotely interested, the cryptic clues that I remember that the Beatles supposedly gave out included:
On Abbey Road album cover - Fake Paul, barefooted but wearing a suit represented the deceased Paul, white-suited Lennon was the clergy man, dark-suited Ringo was the undertaker and denim-wearing Harrison was the grave digger. In the background, a car bearing the number plate 28IF meant Paul would have been 28 IF he had lived.
Magical Mystery Tour album cover, if held up to the mirror (and if you used a bit of imagination), spelled out a phone number that if called supposedly revealed the truth. Turns out it was a Manchester furniture store, and they were mighty displeased with all the calls.
