Until the master was restarted, the ssh connection was freezing:
$ strace -f -o strace.out ssh -vvv gforge
OpenSSH_5.5p1 Debian-4, OpenSSL 0.9.8n 24 Mar 2010
debug1: Reading configuration data /home/vinc17/.ssh/config
debug1: /home/vinc17/.ssh/config line 5: Deprecated option "FallBackToRsh"
debug1: Applying options for gforge
debug1: Applying options for *
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug2: fd 3 setting O_NONBLOCK
debug2: mux_client_hello_exchange: master version 4
debug3: mux_client_request_forwards: requesting forwardings: 0 local, 0 remote
debug3: mux_client_request_session: entering
debug3: mux_client_request_alive: entering
debug3: mux_client_request_alive: done pid = 9656
debug3: mux_client_request_session: session request sent
debug1: mux_client_request_session: master session id: 1
debug2: Exiting on signal 15
Shared connection to scm.gforge.inria.fr closed.
I've attached the strace output (strace.out).
Ctrl-C and Ctrl-\ in the terminal didn't have any effect. However
I could kill ssh with the SIGTERM signal from another terminal.