i have this problem repeatably with my netboot machines.  it happens
if i do any serious work on a filesystem, not just /var or /usr, but
any of my 'user' filesystems as well... (ie, if i use the machine to
build, say src/libexec, it will fail to unmount either /var or the
filesystem /usr/src is really on, but a freshly rebooted machine can
reboot cleanly, no problem).