When using Forms Authentication, and navigating to a page that requires
authentication should redirect to the login.aspx page (or a page specified in the <forms loginurl=""> tag in the web.config file)... .NET will automatically append your login.aspx with ?ReturnUrl=[page the user tried to access un-authenticated].
This is a good read to understand (section: Forms Authentication Control Flow)
http://msdn.microsoft.com/en-us/library/aa480476.aspx[
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