I was working locally and trying to restore a databse backup from the server for development purposes. I was recieving this error when I tried to restore and overwrite an existing databse:
The tail of the log for the database "%ls" has not been backed up. Use BACKUP LOG WITH NORECOVERY to back up the log if it contains work that you do not want to lose. Use the WITH REPLACE or WITH STOPAT clause of the RESTORE statement to just overwrite the contents of the log.
An easy fix (if you aren't too concerned about backups) is to click on the Options page and click 'Overwrite the existing database (WITH REPLACE).
More info here: http://msdn.microsoft.com/en-us/library/bb283410.aspx