There's a part in Stairway to Heaven by Led Zeppelin where if played backwards it sounds like a satanic message. It's ridiculous to assume that Page designed the song with this intent but some people are quite convinced that the song is intended to subliminally induce Devil worshiping.
Here's the part...
If there's a bustle in your hedgerow, don't be alarmed now,
It's just a spring clean for the May queen.
Yes, there are two paths you can go by, but in the long run
There's still time to change the road you're on.
Played backwards sounds like...
Oh here's to my sweet Satan.
The one whose little path would make me sad, whose power is Satan.
He'll give those with him 666.
There was a little toolshed where he made us suffer, sad Satan.
I tried to convince an acquaintance who believed such nonsense that it was pure coincidence but that too was an exercise in futility. I entitled it before I wrote the post and meant to change it later but forgot.
On edit...
Here's a link to the song played backwards..
http://www.reversespeech.com/rev/stairrev.Mp3