I have a couple mount points that would be created by commands
similar to these:

mount_null /foo /mnt
mount_union -b /bar /mnt

Recently, I rebooted the machine that was setup this way and got a
panic: "insmntque into dying filesystem".
(insmntque, getnewvnode, union_allocvp, union_root, union_unmount)
That was with sources from around Oct 14.

I updated to sources as of Oct 26, and the problem was "fixed".

Was this just luck, or does anyone remember actually fixing something
that might have solved this problem?

eric