After upgrading our backup server from 7.2  to Freebsd 8 it refused to boot as it couldn't mount root device. 

ROOT MOUNT ERROR:
If you have invalid mount options, rebott, and first try the following from
the loader prompt:
set vfs.root.mountfrom.options=rw
and the remove invalid mount options from /etc/fstab.
Loader variables:
vfs.root.mountfrom=ufs:/dev/da6s1a
vfs.root.mountform.options=rw
[manual boot prompt]
mountroot> ?
List of GEOM managed disk devices:
Loader variables:
....
##################################

It was weird as booting with old 7.2 kernel somehow worked and at least we managed to boot it in single user mode.

After 2 days of experimenting $#%$#%#$%$#% ..   this solved it:

BSD disklabel lives in the slice, so the one in sector 1 (counting from 0) is the stale one and the one preventing you from booting.

sysctl kern.geom.debugflags=16
dd if=/dev/zero of=/dev/ad6 count=1 oseek=1

(found  on from http://lists.freebsd.org/pipermail/freebsd-current/2009-January/001892.html )