Stairway to Heaven sung backwards
Dutch artist Jeroen Offerman practiced 3 months in a row to learn to sing Led Zeppelins most famous song entirely backwards.
The Stairway at St.Paul’s” is based on the hysteria that surrounded certain music-recordings of the 60’s and the 70’s. Some rock bands, like the Beatles, Judas Priest and Led Zeppelin were supposed to have put hidden messages in their records that could only be heard when played backwards. These messages though, would subconsciously be picked up by the listener who would then react in response to them.
More Info on his website.
More Stairway to Heaven
Here is a list of 101 versions of Stairway to Heaven.
More Info on Backmasking in Music
Here is a list of backmasked messages in music.
Wikipedia has even more backmasking info.