https://sourceforge.net/apps/trac/sourceforge/ticket/22484
There are a number of issues with the registration page that make a pretty bad first impression to the site:
As for the first two points, in my testing, I found that if I edit the "Project Name", and the suggested URL doesn't fit the criteria, it provides a useful error message, but if I edit the URL field directly, and the URL doesn't fit the criteria, the page doesn't notify of the issue until submission (then it behaves just as the user describes).
As for the third point, perhaps pointing to this doc would be appropriate somewhere?
Validation is better now. "Classic Project" availability will be removed over time, not going to document the difference on the reg page.