The only TRUE, without a doubt backmasking I've ever heard that I am 100% convinced was done intentionally and is actual words, is the backmasking Pink Floyd did in some of their songs...none of it is evil or anything...infact, it's humorous.
I'm not entitely sure, but Marilyn Manson has probably done it intentionally. Not that I'm complaining. It's one of my favorite bands. It's just that on Anti-christ Superstar there are some parts that when played regular sound like something is being said in reverse, so if it was played in reverse real words would be heard. I've never figured out a way to test it out though.

Tell me, how did you play zepplin backwards?