If you use the current „head“ Version (Release 12) of FreeBSD with a svn updated /usr/src path you could run into errors after updating the OS via buildworld from sources.
There’s a bug ahead Revision „r314099“ at the „portsnap“ lib that will stop on extracting the ports.tgz to /usr/ports!
Solution: Pull again sources to /usr/src with
$svn update /usr/src $make buildworld $make buildkernel $make installkernel $reboot $mergemaster -p $make installworld $mergemaster -FiU $reboot $portsnap fetch update #again.. to update ports tree $portupgrade -a $reboot
This should work now, the failed portsnap extract function was a known bug. If you dont want to rebuild the system, you can use svn to update /usr/ports via svn repository. You can later rebuild the source OS again.