sporadic disk syncing failures when shutting down

[Don Lewis]

I've been updating my current system a lot recently, and twice in the
last couple of weeks, the disks have not been properly synced before the
system reboots.  I've been doing the usual
	make buildworld
	make buildkernel
	make installkernel
	shutdown -r now
	make installworld
	mergmaster
	shutdown -r now
procedure and the problem has always happened on the final shutdown.

 Stopping inetd.
Shutting down daemon processes:Jul 12 09:19:40 scratch upsmon[492]: upsmon parent: exiting (child exited)
killall: warning: kill -TERM 492: No such process
killall: warning: kill -TERM 488: No such process
 healt Waiting (max 60 seconds) for system process `vnlru' to stop...stopped
Waiting (max 60 seconds) for system process `bufdaemon' to stop...stopped
Waiting (max 60 seconds) for system process `syncer' to stop...stopped

syncing disks, buffers remaining... 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 
giving up on 54 buffers
Uptime: 6m42s
Terminate ACPI
Rebooting...

Each time this has happened, fsck finds and nukes a bunch of empty
directories.  The last time this happened, the /etc/rc.d/yp* files that
mergmaster updated were missing after the reboot and fsck had done its
work.  Nothing has ever shown up in lost+found.

Has anyone else seen this?