When I was about 14/15 my best friend and I used to play the Beatles records backwords looking for backmasking. We found the classic "Paul is dead", "Paul is buried in the cellar" ones and others. Supposedly if you turned Sgt. Pepper's upside down and then read the bushes that had spelled THE BEATLES you would get a phone number to the London morgue and someone would whisper "You're getting closer." from the other end and hang up. We tried it but the number either didn't exist or we couldn't reach it.
I also liked the Funeral Procession theory for the Abbey Road cover. Paul is dead symbolized by being barefoot and smoking a "coffin nail". Ringo is the grave-digger wearing the jeans and work type clothes. George is a pall-bearer and John is the priest dressed all in white. Also, if you look into the background there is a VW with the license plate reading "28-IF". Paul would have been 28 years old at the release of Abbey Road had he not been dead.