So, I got a new bike today. I tried adjusting the derailleur because it was a bit off. It was the first time I've ever tried it, but I think I did a pretty reasonable job on it. The only problem I now have is that whenever I backpedal in first gear (and only in first gear), the chain will slip down into second gear. It doesn't slip if I backpedal on any other gear, and when pedalling normally the gear changes are all smooth. Why is this happening?