Backmasking is when you play a song backwards. And people discovered this and started doing it years ago. Some songs have hidden messages while others might just be a coincidence.
One song I listened to backwards was stairway to heaven. It was a huge paragraph that was played backwards so I think they meant to say things about satan backwards.
Another one was the Britney Spears song "Baby one more time". I have talked to some people about this one and they think she said it the reverse way first then thought it sounded bad. Yet, when she played it backwards it said "with you I lose my mind" and she decided to keep it that way. Thus, I think she meant to
have that one.
The song "beat it" by Michael Jackson says "tell you its fair" then backwards it says "I believe it was satan in me" If you really listen its not too hard to figure out what hes saying but it is a little difficult. I personally do not think he meant for that one to happen.
The song "Another one bites the dust" says that repeatedly in the chorus. Backwards it says "It's fun to smoke marijuna." I think because it is a very long chorus of just that I think he meant that one. Plus, you could hear it clearly.
The song "Kiss Kiss Kiss" by Yoko Ono says "Anata Daiteyo." Backwards though it says " I shot John Lennon" This is ironic because that was her husband and he was assassinated. I honestly do not think she meant for that to happen because she loved him and I don't think she would want to say that.
"Revolution 9" by the Beatles repeatedly says "Number 9." Backwards it says, "Turn me on dead man." I think they meant this because there was a whole conspiracy with Paul McCartney being dead and they wanted to go along with it. Also, it was a long paragraph so they must have known and you can hear it clearly.
The Doors song "Break on through" says, "Treasure's there." Backwards it says "I am satan" I think they meant to do this. Even though its a short line you can hear it pretty clearly.
"Nature tail to hell" by Weird Al is very hard to hear the words forward so you can't make out what he's saying. Backwards it says, "Satan eats cheese whiz." I think he meant to do this because Weird Al would do something like that. Plus, it is pretty clear.
The song "My name is" by Eminem is just the chorus. Backwards it says, "It's Slim, It's Eminem" and keeps repeating it. I think he meant for that to happen because it is too much of a coincedence.
In John Lennon's song "Imagine" he says, "Imagine all the people." Then, backwards he says, "The people war beside me." I think he did not mean for this because it doesn't make much sense. Also, he didn't like war.
I got all of these from Jeff Milner's site. I based on if I thought they meant to put it there based on how clearly you can hear them say the words backwards.