I have nicked this from the bbc website, but I thought it was pretty interesting.
It's basically pointing out how once you've noticed something in a song, you can't "unhear it" (it jumps out at you every time you listen)!
A quick, but awesome demonstration (on soundcloud) is here:
https://soundcloud.com/whyy-the-pulse/an-audio-illusion
If you want to read to full article... clicky clicky:
http://www.bbc.co.uk/music/articles/...5-732d067c7e9e