Why do I get a 'The process cannot access the file because it is being used by another process.' error?

This is a common problem encountered when the torrent client is attempting to write to a file that is presently being accessed by another process or application.  Close all applications that could possibly open such a file, including Finder and Explorer windows. If this continues to occur, restart the torrent client.  Restarting your system may be required as well.