After upgrading to SP4 on a 6.5 box, my BM picked up old filters, presumably
from an old filters.cfg file that had been renamed to filters.old, but I'm
not sure.

Fortunately, the problem was easy to fix: restore latest version of
filters.cfg from backup, unload filtsrv.nlm and then run 'filtsrv migrate'
from the command line. That will copy the filters from filters.cfg into
edirectory. Then, unload filtsrv.nlm again and 'reinitialize system.'
Voila - all fixed. Just make sure you do a backup before running the
support pack (as you should, anyway!).

Hope this saves somebody from some of the confusion I went through ...
- J