We’ve had huge problems with the testserver. It first felt like a cache problem, because when you updated the content and hit refresh several times, you saw the changes and then you didn’t. Also, pages that were dependant on session state were giving null expressions since the state was empty sometimes.
The problem was in the Application Pool. DefaultAppPool was for some odd reason by default set to 3 worker processes which makes it a web garden (used for load-balancing) and this caused the user to get the errors mentioned above, since only one of the worker process actually had a valid session, the others didn’t.