If you are using the latest version of Umbraco on a site that is
using the default membership provider, you may have stumbled into
the problem of users not being logged out properly.
I recently had this issue when using nForum on a site, and when
the user logged out they appeared to still be logged in - This is
something that the core team are aware of and it will be fixed in
the next version, but if you need a work around the following code
works to log the user out fully.
Member.RemoveMemberFromCache(Member.CurrentMemberId());
Member.ClearMemberFromClient(Member.CurrentMemberId());
FormsAuthentication.SignOut();
Roles.DeleteCookie();
HttpContext.Current.Session.Clear();