FreeBSD bsdinstall(8) Checksum Errors
If you ever get a checksum error during the download of any of the distribution packages for a bsdinstall(8)-ation, try to re-fetch the files for your current version (freebsd-version(1)) manually and try again.
# cd /usr/freebsd-dist
# for f in MANIFEST base.txz lib32.txz;do fetch http://ftp.freebsd.org/pub/FreeBSD/releases/amd64/$(freebsd-version)/$f; done
You should be good to go!