We are experiencing random intermittent session failures.  This means the user has been “logged out”  If they refresh thier page they will get run through the login screen and put back on that page.   Unfortunately they will lose changes made on the page that had not be saved.  Boo…   This is pretty thorny issue because Salesforce controls the session information and we can’t do much about it. But we are paying attention to it and trying to see what can be improved…