If your system is far out of date you may encounter this error:

/usr/lib/portage/bin/ebuild.sh: line 370: xz: command not found
tar: This does not look like a tar archive
tar: Error exit delayed from previous errors

This is due to the switch from lzma-utils to xz-utils last year as covered in this post: Portage Errors: app-arch/lzma-utils is blocking app-arch/xz-utils. As covered earlier it is safe to remove lzma-utils then emerge xz-utils because it ships in a traditional (gzip) tarball:

# emerge --unmerge lzma-utils
# emerge xz-utils