df shows negative free percentage

[Jeff Dickens]

I just noticed one of my FreeBSD 6.2 systems showing "-0%" free in 
/tmp.  If I copy a file to it, it shows a sensible percentage free, but 
then reverts to "-0%" when I delete that file.  Any idea what's going on 
here?

    FreeBSD 6.2-RELEASE (GENERIC) #0: Fri Jan 12 10:40:27 UTC 2007

    Loon

    loon# df
    Filesystem  1K-blocks    Used   Avail Capacity  Mounted on
    /dev/da0s1a    507630   38436  428584     8%    /
    devfs               1       1       0   100%    /dev
    /dev/da0s1e    507630     -20  467040    -0%    /tmp
    /dev/da0s1f   5366090 1573304 3363500    32%    /usr
    /dev/da0s1d   1255886  356994  798422    31%    /var
    loon#
    loon# cp /usr/local/kits/vmware-freebsd-tools.tar.gz /tmp
    loon# df
    Filesystem  1K-blocks    Used   Avail Capacity  Mounted on
    /dev/da0s1a    507630   38446  428574     8%    /
    devfs               1       1       0   100%    /dev
    /dev/da0s1e    507630   10444  456576     2%    /tmp
    /dev/da0s1f   5366090 1573304 3363500    32%    /usr
    /dev/da0s1d   1255886  356994  798422    31%    /var
    loon# rm /tmp/vmware-freebsd-tools.tar.gz
    loon# df
    Filesystem  1K-blocks    Used   Avail Capacity  Mounted on
    /dev/da0s1a    507630   38446  428574     8%    /
    devfs               1       1       0   100%    /dev
    /dev/da0s1e    507630     -20  467040    -0%    /tmp
    /dev/da0s1f   5366090 1573304 3363500    32%    /usr
    /dev/da0s1d   1255886  356994  798422    31%    /var
    loon#