This is the weirdest thing that has ever happened to me.
I am working on a project animating lower thirds, creating the animations over a small movie clip. Now every time I create a new Composition, it comes with that video clip as if it was the background/first layer, but there is no layer, there is nothing. I haven't add anything to the composition yet!!!
Purge all memory and disk cache. You'll find it under the edit menu.