Geek alert: skip this if you don’t administer Open Directory on OS X Server Leopard (10.5.2 thru 10.5.4 as of this writing).

Regarding this Open Directory critical bug (Apple problem ID #5935780):

If you happen to have a terminal window open with root logged in you can killall -ILL DirectoryService to force OD to crash and restart (and shame for leaving a root shell open :-).

Slightly more sane is a regular launchd job to prophylactically restart OD using the same command.  Lingon is a great resource if you don’t know how or want to bother figuring out how to create a new launchd job.

If you happen to experience this bug, PLEASE report it to Apple!