I notice that large sites like Facebook and Twitter redirect users to alternative login pages if they do not enter their login credentials properly. I'm still fairly new to SEO, but my UX knee-jerk reaction is that this is probably done for a reason, since the better experience clearly would be to keep the user on the first page they were on and append them with a bad username/bad password message (like what stackexchange does).My reasoning is; since logins are a form, pushing to a new page after any submit  proper login or not  might be an easy way to squeeze more hits out of a site.I'm wondering if I'm correct about this, as there are likely many other reasons for redirecting on a bad username and password. 
How do form submissions affect PageRank?
seo;redirects;pagerank
Although each linked and visited page counts, I don't think anybody really consider that traffic as relevant, except for some kind of problem, like a poor form, a bad validating system, an attack, or any other alternative.Some sites give more value to UX, those give clear instructions to the user, before filling the form and after a mistake.Some sites give more value to security, so they implement a special page and process if you make a mistake filling, for instance, a user/password form.Some sites offer combinations of the above.If your site is big enough, who cares how many times that second page is accessed; on the other hand, if your site is small, you should care about attracting visitors with useful content.