Production Librarian garbage collection failing

There is rubbish in the production Librarian storage area. The code that ignores this rubbish is broken, causing garbage collection to fail with an exception:

2009-09-25 17:53:08 ERROR Ignoring directory incoming.old that shouldn't be here
Traceback (most recent call last):
 File "/srv/launchpadlibrarian.net/production/launchpad2/cronscripts/librarian-gc.py", line 93, in ?
   script.lock_and_run(isolation=ISOLATION_LEVEL_AUTOCOMMIT)
 File "/srv/launchpadlibrarian.net/production/launchpad2-rev-8513/lib/lp/services/scripts/base.py", line 286, in lock_and_run
   implicit_begin=implicit_begin, isolation=isolation)
 File "/srv/launchpadlibrarian.net/production/launchpad2-rev-8513/lib/lp/services/scripts/base.py", line 244, in run
   self.main()
 File "/srv/launchpadlibrarian.net/production/launchpad2/cronscripts/librarian-gc.py", line 87, in main
   librariangc.delete_unwanted_files(conn)
 File "/srv/launchpadlibrarian.net/production/launchpad2-rev-8513/lib/canonical/librarian/librariangc.py", line 548, in delete_unwanted_files
   dirnames.remove(dirname)
ValueError: list.remove(x): x not in list