I don't consider that a bug. Its 2 ways of chossing what to map to. Pages only have one url attached to them that they know about if there happen to be other mappings to the page they don't know about it. If you choose the page it maps to the one it knows about. So the page list dropdown has no idea whether something you typed maps to a page in the list.

The Url Manager is buried beneath Advanced Tools for a reason, its generally best not to muck with that and let the system manage the urls for you. If you do it yourself you really need to understand whats going on with the url mapping or you can easily break things.

I don't recommend to use folders and default.aspx files except for very specific things in a multi site installation based on folders. The url system is designed not to reflect site hierarchy on purpose. Urls should be short and should convey the title of the content. This way you are free to move pages around wiothout breaking urls. Good urls don't change. Its also best not to have more than one url pointing to the same page from and seo persepctive.

Urls should end in .aspx unless you have some custom isapi filter on IIS 6 or some custom module in IIS 7.

Hope it helps,

Joe