I used to play some songs backwards on my first computer.
I remember, when I was just a wee laddy - I'd go to a youth group put on by a local Church. The priest or whatever was actually a pretty cool guy, who liked hard rock. Anyways, he had a tape that he had been given, with a lot of hard rock song bits played backwards. It was put together and distributed to anyone who would take it, to show the evils of Rock and Roll that are brainwashing the children.
We used to listen to them, partially laugh about the people that went to all the trouble to find them - but we all agreed, it was spooky and cool. Not what the tape makers intended, I'm sure.
Then that thing on MTV came out saying the 2 most satanic bands were AC/DC and Iron Maiden (yes, this was the 80's) - and I got stared at in this small town because the only shirts I had at the time were AC/DC and Maiden shirts.
There are a few songs by Black Sabbath, AC/DC, Iron Maiden, Rolling Stones that you can hear something on when played backwards, I think it was likely just a lucky coincidence - I don't think any of those bands would be smart enough to do it on purpose.
I like the freaky story about the song Black Sabbath, and how it was written. The band, staying in some old castle for recording to get a mood, were all in bed, all in seperate rooms. All of them saw some weird black cat human figure, which scared them very much. The next morning, they picked up their instruments, and played the song Black Sabbath for the very first time, but all knew the song already - some how.....
Iron Maiden, when recording # of the beast (I think it was recorded somewhere weird too) had either the song or the whole albulm get weird sounds on it after it was recorded, and they had to do it all over again. That freaked them out too, if I remember correctly.