'"Backmasking" refers to the fact that human speech played backwards still sounds like human speech, albeit stripped of its original meaning, a fact much exploited to create "alien" speech in video games such as Heretic. The power of pareidolia (the tendency of the human mind to see more detail than is actually present, especially if actively looking for it), coupled with that of coincidence, means that surprisingly often, the "new" speech still appears to have meaning and even significance. The best-known Beatles example is probably the "number 9..." loop from "Revolution 9"; the "Paul is dead" hoax claims that when this is played backwards, it becomes "turn me on, dead man" (and now that you have been told this, if you try the experiment this is what you are likely to hear), but other sources beg to differ.
If you think about it, to do this sort of thing deliberately would be extremely difficult if not actually impossible, like trying to compose a written (and meaningful) sentence which becomes another (still meaningful) sentence when written backwards.
- Backmasking on Wikipedia