there are definitely songs that do that, but it's 100% intentional. if it ever happend by accident, i would throw it in there as bullshit also and say it was staged.
i had a walkman (i'm old) in high school that had 2 buttons that you could use to flip from one side of a tape to another, i took out the spring so i could hold the buttons half way and listen to stuff backwards (when i first heard about this stuff, i think it was from the movie "gate").
anyways, it would play both sides of the tape at the same time, so i recorded all my slayer albums to a tape with nothing on the other side. particularly the song "hell awaits" intro, says "slay god" in a creepy whispery way for a while, when you play it backwards it definitely says "join us". there's all kinds of shit like that in slayer chorus in all kinds of songs. but again, that's their gimmick, and it's 100% intentional.
