I had never heard it, but heard of it

Reverse speech is a common occurence in everyday life. In fact, many scientists believe that babies first learn to speak backwards (goo goo ga ga? anyone?).

www.reversespeech.com

It can be done as a way of reaching the mind subliminally, or it can come out of you without you noticing it.

People form interesting habits in speech, I for instance speak in Iambic Pentameter like 60% of the time. I had no idea till someone pointed it out to me and I freaked out.