##  Enable auto completion even when needing a password
## as long as one connnection to the server has been established.
Host *
  ControlPath ~/.ssh/master-%r@%h:%p
  ControlMaster auto

$ ssh -vYp 1234 terdon@123.456.78.9
OpenSSH_6.2p2 Debian-6, OpenSSL 1.0.1e 11 Feb 2013
debug1: Reading configuration data /home/terdon/.ssh/config
debug1: /home/terdon/.ssh/config line 3: Applying options for *
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: auto-mux: Trying existing master
debug1: mux_client_request_session: master session id: 14
Last login: Tue Oct 15 20:12:24 2013 from foo.bar.net
terdon@server $ echo $DISPLAY

terdon@server $

debug1: Requesting X11 forwarding with authentication spoofing.
debug1: Sending environment.
debug1: Sending env LANG = en_US.UTF-8

terdon@server $ echo $DISPLAY
localhost:11.0

Host *
  ControlPath ~/.ssh/master-%r@%h:%p
  ControlMaster auto
  X11Forwarding yes

X11Forwarding yes
Host *
  ControlPath ~/.ssh/master-%r@%h:%p
  ControlMaster auto

Bad configuration option: X11Forwarding

$ lsb_release -icd
Distributor ID: LinuxMint
Description:    LMDE Cinnamon Edition
Codename:       debian
$ uname -a
Linux oregano 3.10-2-amd64 #1 SMP Debian 3.10.5-1 (2013-08-07) x86_64 GNU/Linux
$ ssh -V
OpenSSH_6.2p2 Debian-6, OpenSSL 1.0.1e 11 Feb 2013