The problem occurs when using the user login module/menu item to redirect a page. By default index.php becomes the default, so even if you leave the fields blank you will generally still see instead of

This tends to break the session thus the stability of your view of the website, usually a white screen of death scenario or token invalid results, frustrating users who usually threaten to abandon using the offending website. The way around this is to add a meaningful redirect into the fields of the previously mentioned extensions or just add a forward slash (/), no brackets of course.