Google threw one of my sites back in the chitbox when I did a major re-design of a site. I somehow forgot to add the php code that added title tags to a few thousand pages, and google began to use the url as a title in the search results. Traffic dropped like a rock and googlebot really threw up when I finally added the title tags.
Took about three months before all my pages were re-indexed, and back to their original search rankings. Luckily, not many pages went supplemental.