If the user clicks the Version Control tab before the CVS information was retrieved the thread just hangs until the CVS repository is parsed. If the repository takes a long time to parse or retrieve teh log the UI thread is locked up. Many users just hit refresh and lock up another thread.
This is a huge problems when the CVS server stops responding as the thread takes a long time to time out.
In JIRA 3.4 this is not too bad as there is a timeout on CVS checkout thread. However, the timeout is quite long (10 minutes) so problems still arise.