The beauty of the InterWeb is that sometimes you re-discover old things.
Back in the old days, people accuse Black Sabbath of sneaking satanic messages into their song (that you can only hear by playing it backwards).
That's nothing compared to Aphex Twin, who sneaked in the devil's face in their Windowlicker EP in 1999. The face is revealed when the song is played by a computer software that visualizes sound waves.
Link | Wired Article in 2002 - Thanks Dougall Meloney!