Design is an iterative process. Sometimes (most times) you will need to go back to the previous step. On some occassions the project will be nearly complete when you need to start again. The process looks like a waterfall that can flow upstream.

Also see http://www.foylearts.net/ahutton/mobile/?p=418

http://en.wikipedia.org/wiki/Iterative_design

Also see this http://simplerisbetter.wordpress.com/2009/01/15/context-is-everything/  and read about the sad story of the SA80