Today I was facing a very special problem with IE8 (compatibility mode) and IE7 when a user logged in the frontend using SSL (https). The login worked seamlessly, the first page after the login was correctly shown. But on his next cklick the user got a Page not found error because he has automatically been logged out. Inspecting the traffic showed that there was a single ressource in the page (favicon.ico) which was requested but was not available on the server. On top of that this ressource was for some strange reason requested via http. These two things were the reason why the user has been logged out.