When you try to update your ports tree by running the command:
# portsnap fetch update
After some files you will get a error message:
snapshot is corrupt
This is probably due the fact that the portsnap fetch update command hasnt been executed for some time,
and that it needs to get a lot of updates.
# rm /var/db/portsnap/tag && portsnap fetch extract
This will download a complete new ports tree.