It's come up before where somebody on this forum took home a new car and then the dealer called a few days later and said the financing fell through so bring it back. Here's an article about the entire tactic that dealers use that causes this. Really good read on CNN Money site.
http://money.cnn.com/2004/01/14/pf/autos/yoyo/index.htm
http://money.cnn.com/2004/01/14/pf/autos/yoyo/index.htm