Flow is arguably one of the most important aspects of any UI, and it's even more important on the web. With latency measured in seconds, not being taken to the right page can have devastating effects on the user experience.
I was recently buying movie tickets on fandango.com (which I don't like to do -- stupid surcharges) and couldn't remember if I'd ever been to the site before. I added my tickets to the shopping cart and tried to create a new account so I could check out, and this is the page I got:
Hrm, common sense dictates that if an account exists with the e-mail address, then either one of two things happened: I made a typo or I forgot I already had an account. There should be links there that offer to send a password reminder to me or allow me to login with the e-mail address I'd just entered. That would have made for a seamless exerience.