How many do this from time to time. You play the song in reverse and listen for subliminal messages, its quite fun but creepy at times.
A classic example would be Stairway to Heaven by Zeppelin.
http://backmaskonline.com/classicrock.html-- here are some classic songs reversesd.
If you want to backmask your own songs on the computer, convert the mp3 or whatever format you use into WAV. Open up soundrecorder and open the WAV in soundrecorder. On the effects menu, click reverse and play
Backmasking songs
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