Mac OS X Lion Terminal SSH connection sharing error

Question

I sometimes get this error when I try to ssh to a remote host:

mux_client_request_session: read from master failed: Broken pipe

I'm using Macbook Air OS X Lion with this in ~/.ssh/config:

ControlMaster auto
ControlPath /tmp/ssh_mux_%h_%p_%r

ControlPersist 4h

ForwardAgent yes

Host [ex]
    HostName [example.com]
    User [somedude]

The error above happens when I do:

ssh ex

Terminal hangs for ~1 min and then displays:

mux_client_request_session: read from master failed: Broken pipe
somedude@example.com's password:

I type in my password and it's ok. And if I open another Terminal tab, connection sharing is ok too. How can I fix this? It's not terrible, but it's annoying.