If   local_dir  is in an NFS share, the mount command fails:

sshfs  remote_host:remote_dir  local_dir
fusermount: failed to open mountpoint for reading: Permission denied

This could be avoided by changing the permissions on  local_dir , but then
there are problems with the unmount step, as described in

http://superuser.com/questions/290542/how-to-cleanly-dismount-sshfs-when-
mounted-on-a-nfs-mount-point

It appears that the only solution is to place  local_dir  somewhere in a
local filesystem. If this is so, it should be documented e.g. in README.Debian
at least.