Though you may associate Easter eggs in music with metal bands, the idea of songs with a hidden or subliminal message spans all genres. While the usual suspects, such as Ozzy Osbourne and Judas Priest, make the list, the most famous instances of adding messages into songs come from the Beatles - fans claim you can find hidden messages on several Fab Four songs.
A few of these secret messages supposedly caused people to commit suicide; others prompted conspiracy theories that remain popular today ("Paul is dead," anyone?); others still are just plain creepy. And then there's the usual "backwards-masking" calls to Satan, of course.
Hey, even Prince and The Eagles supposedly put hidden messages in their songs. You can check out of the Hotel California any time you like, but you can never leave - particularly if you listen to classic rock radio.