Stop in /tinderbox/CURRENT/i386/pc98/src/sbin.
*** Error code 1
When what previously was a file (sbin/ipf/ipf) becomes a directory,
the old .depend file gets in the way (SUBDIR's use the :: operator).
Someone needs to remove the stale .depend file, or wipe out the
obj directory completely, or this particular tinderbox will fail
FWIW, removing the ipf directory under /usr/obj such like
"rm -r /usr/obj/usr/src/sbin/ipf" did the trick for me.