The Resource Cannot Be Found

Hi All
I'll explain the sequence of events first.
 
1. Login
2. Select an option from Main Menu.
3. Select Main Menu.
4. Select Log out (from Main Menu).
5. Press browser back button.
6. Following message is displayed:-
 
  Server Error in '/' Application. The resource cannot be found. Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable.  Please review the following URL and make sure that it is spelled correctly.

Requested URL: /Web Pages/Login.aspx

Version Information: Microsoft .NET Framework Version:2.0.50727.4206; ASP.NET Version:2.0.50727.4205

 
...