This thread gets into two different concepts. I will attempt to address them both.
First and most easily dismissable are the people who just play stuff like George W. Bush speeches backwards and picking out things.
Now, the fact is that almost any human speech played backwards will resemble something - especially if you REALLY want it to.
This is because you have two principals involved. Both the fact that the nature of spoken communication means that something said backwards has the potential to resemble something else, and the nature of human beings to hear things that they want to hear collide into a vast conspiracy.
So, while it does not surprise me that there are words which can be heard when played backwards in speeches, it means nothing more than the fact that we can see things in clouds, and sometimes we need someone to tell us what THEY saw in them before they see it themselves.
Same principal.
The second concept is purposely putting something on a recording that is put to the music backwards. This is an easy studio trick that anyone with the basic sound recorder that comes with Windows can replicate on their computer.
The problem here is that most all of the cases that have come to the public’s attention are cases of the first example above, just set to music.
I remember hearing a member of Judas Priest saying, “It really does sound like we’re saying 'Do it,” doesn’t it?" in fact, during the trial. But they claimed, and I (and most people) believe them, that this was not something intentional.
Even if they did put it in there intentionally, there is no proof that backwards masking has any effect anyone on people. So, Britney Spears could have “Kill yourself now! You don’t deserve to live” intentionally put backwards into “Oops…” and it would have no effect anyway.
Now, as I said, someone could put a real backwards message on an album - it would stand out, though, the same way that listening to anything backwards has a quality that makes it stand out.
My favorite was on Overkill’s Feel The Fire debut disc from 1985. At the end of the last song on side two, you hear some gibberish. When this gibberish is played backwards, you hear the following:
“There’s no message here! You’re going to ruin your needle, asshole!”
Yer pal,
Satan
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