I have ran into an issue recently with Quality Center 9.0. Quality Center 9.0 will let a user log into the system with the username in either lower or upper case. Now if a user checks a test out with their lower case username and modifies the test, then logs out. Next time the user loggs in, they use uppercase letters. When they try to modify, or check in the test, the user recieves a message stating "Cannot perform update if test is not checked out by current user"
If I remember correctly, in QC 8.2, the login was case senesitive, meanign that if a username did not match what was defined in the system, then they were unable to login.