There's a recent Bryan Adams song which copies lines wholesale from Dire Straits' Romeo and Juliet. It was blindingly obvious to me, but a friend hadn't noticed it until I pointed it out. I'd guess it's like an optical illusion, or the Stormtrooper banging his head on the door in the original Star Wars - you can't see it at first, but once it's been pointed out, or once you've spotted it in a moment of clarity, suddenly you can't help seeing it all the time.

Grab.