It occurred to me that Michael D. Spence wrote in gmane.os.netbsd.general:
> Version 1.6:
>
> disks:
> Filesystem  1K-blocks     Used     Avail Capacity iused   ifree  %iused
> Mounted on
> /dev/wd2e     7986648  7901839   -314524   104%   99954 1931660     4%

By default, 5% of the space inside a filesystem is reserved for use by
the superuser. See tunefs(8), especially the "-m" flag.

So this probably indicates that it's root's own fault that the filesystem
is full :)