4.3 BSD VAX 11/750 does not seem to sync its disks.

[Ron Natalie]

Don't run fsck on mounted and busy file systems.  You'll destroy
things in progress.  Generally, one should never run FSCK on a
mounted filesystem at all (except for the root which you have
no choice).  There are several causes of unreferenced files that
are still in use, pipes, and certain programs will create them.
Blowing them away with an FSCK is a BAD idea.

-Ron