I normally don't do SEO for clients and on the rare occasion they seek me out and convince me to, I make sure they sign an iron clad agreement that states " I guarantee nothing, other than I will do what has worked for me since I started doing it." No first page, nothing,.. That way, when crazy things happen as they do from time to time for no reason, I can say that's why I don't guarantee anything as far as SEO goes.
SEO is not the rocket science many make it out to be.
I really love when people call me and offer SEO services. I have a lot of fun with those calls :)