I've updated my comment for clarity. The bug (which I've never encountered in more than 20 years as a Thunderbird user) is that users move messages to a local email folder, but the messages are deleted from the server without actually downloading them. At a minimum they should disable that operation. The guy that originally reported it worked at Sun and lost hundreds of work messages as a result of this bug. AFAICT the user wouldn't be affected if they did a copy of the messages and then manually deleted them from the server folder after confirming the copy was successful.
