When forms give users the option to continue in two or more alternative directions, such as registering as a new customer or signing in as a returning one, unfortunate users will take the wrong turn if it isn't unmistakably obvious which way they should go. In this article, we'll take a look at a few intersection flows that have caused users problems.
Olsen, Henrik. GUUUI (2005). Articles>User Interface>User Centered Design>Workflow