i already posted this elsewhere but here goes:
(I have read the following online and have not tried to confirm its authenticity yet)
There was a case brought in the US...sometime in the '80s i think...against the bands or record companies of bands of one or two heavy metal bands, including Judas Priest. There had been a few teenagers who had been heavily into their music who had gone and committed suicide. Anyway the claim was that these bands had hidden backwards messages in some tracks urging the listener to commit suicide.
As part of the defense, one particular track (it may have been of a completely different band to those who were the defendants) was played backwards to the jury. Then, the jury were given a copy of some words that could supposedly be heard in the track, then the track was played again.
The point of this was to demonstrate that, once you have been told what message can be heard in a piece of music being played backwards, it is alot easier for you to hear that message, whereas listening to the tune backwards without knowing what you're looking for will not allow you to hear words, unless there really is a hidden message that has been backwards masked (as the beatles did in some of their tracks)
In short, i have listened to Stairway To Heaven backwards with a copy of the supposed entire song's worth of backwards-masked words that are supposed to be some kind of "black mass" or something. My personal opinion is that whilst there may be a short message hidden in there, i do not agree with there being an entire song's worth in there.
On a similar note, as i also mentioned in the other thread, i have personal experience of the effects of psychosis, a mental disorder that alters your perceptions, sometimes drastically. I'm sure there are many songs that, if you listened to them backwards in that state, you'd be able to make out words that seemed to make a certain sort of sense. This is what i think happened to produce the song's worth of supposedly backwards-masked words that can be heard. When i was somewhat worse than i am now (well on the mend, thankfully), i heard different words that still made sense when listening to songs FORWARDS, so i am sure if i'd have listened to them backwards it would have blown my mind...