> /usr/src/sys/arch/mac68k /home/paul/slotman-backup null  rw     0       0
> /home/paul/slotman       /usr/src/sys/arch/mac68k  union rw     0	0

Thanks.

Ok, I've reproduced a failure (though not exactly the same as yours)
with:

mount -t null /usr/b /usr/a
mount -t union /usr/c /usr/b

I'm working on some union-related locking bugs at the moment, so I'll
take a stab at it..