The "dev" package's preinstall script does the following:

/usr/sbin/groupadd -g 19 -r -f floppy > /dev/null 2>/dev/null ||:

This groupadd is necessary because files like /dev/fd0 will be installed
under this group. However, this script, as written cannot run if "dev" was
never previously installed, because /dev/null doesn't exist yet! It's the
classic chicken-and-egg problem :)

This problem is never visible during normal installations, perhaps because
of
the way the standard Redhat installer works (maybe it creates a simple /dev
when
starting - I don't know). But it did present itself whenI tried to create a
"custom"
distribution by opening up a list of rpms myself using something like
        rpm -root $DIR file.rpm ....

Possible solutions are to remove the redirection (perhaps a "-q", quit flag
should be added to the groupadd program?), to do an mknod of /dev/null
in the preinstall script (this is probably ugly), or to move /dev/null to
another
package (e.g., "setup").