There was a soft updates bug that crept into 4.5-RELEASE, and could cause corruption on shutdown if there was heavy disk activity before the shutdown took place. This hasn't shown up on any lists I read, but it seemed a bit more noteworthy than the latest security hole in some obscure port (no, not PHP).
From the errata page: A bug has been fixed in soft updates that can cause occasional filesystem corruption if the system is shut down immediately after performing heavy filesystem activities, such as installing a new kernel or other software. The system shutdown was unable to flush all buffers on shutdown and would report this fact. The problem can be worked around by running sync(8) a few times before rebooting, or solved by updating to a recent FreeBSD 4.5-STABLE snapshot.