Disabling GSSAPI Key Exchange in the client config makes it proceed as expected, so that is a workaround.

Deleting the moonshot mech file doesn't change the behaviour, so it would seem that the GSS patches are at fault, rather than the presence of our mechanism directly.

[adamb@ms-ssh-rp ~]$ ssh -vv -l "" ms-ssh-rp.dev.ja.net
OpenSSH_5.3p1, OpenSSL 1.0.1e-fips 11 Feb 2013
debug1: Reading configuration data /etc/ssh/ssh_config
debug2: ssh_connect: needpriv 0
debug1: Connecting to ms-ssh-rp.dev.ja.net [212.219.210.74] port 22.
debug1: Connection established.
debug1: identity file /home/DEV/adamb/.ssh/identity type -1
debug1: identity file /home/DEV/adamb/.ssh/identity-cert type -1
debug1: identity file /home/DEV/adamb/.ssh/id_rsa type -1
debug1: identity file /home/DEV/adamb/.ssh/id_rsa-cert type -1
debug1: identity file /home/DEV/adamb/.ssh/id_dsa type -1
debug1: identity file /home/DEV/adamb/.ssh/id_dsa-cert type -1
debug1: identity file /home/DEV/adamb/.ssh/id_ecdsa type -1
debug1: identity file /home/DEV/adamb/.ssh/id_ecdsa-cert type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
debug1: match: OpenSSH_5.3 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.3
debug2: fd 4 setting O_NONBLOCK
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: <email address hidden>,<email address hidden>,<email address hidden>,<email address hidden>,ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,<email address hidden>
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,<email address hidden>
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,<email address hidden>,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,<email address hidden>,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,<email address hidden>,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,<email address hidden>,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,<email address hidden>,zlib
debug2: kex_parse_kexinit: none,<email address hidden>,zlib
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: kex_parse_kexinit: gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-eipGX3TCiQSrx573bT1o1Q==,gss-group14-sha1-eipGX3TCiQSrx573bT1o1Q==,gss-gex-sha1-eipGX3TCiQSrx573bT1o1Q==,gss-group1-sha1-JHo1wNMhky/7DkDu4d6xCA==,gss-group14-sha1-JHo1wNMhky/7DkDu4d6xCA==,gss-gex-sha1-JHo1wNMhky/7DkDu4d6xCA==,gss-group1-sha1-dEYdZI86nhHqawDlBMslQw==,gss-group14-sha1-dEYdZI86nhHqawDlBMslQw==,gss-gex-sha1-dEYdZI86nhHqawDlBMslQw==,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,<email address hidden>
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,<email address hidden>
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,<email address hidden>,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,<email address hidden>,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,<email address hidden>,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,<email address hidden>,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,<email address hidden>
debug2: kex_parse_kexinit: none,<email address hidden>
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: mac_setup: found hmac-md5
debug1: kex: server->client aes128-ctr hmac-md5 none
debug2: mac_setup: found hmac-md5
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug2: dh_gen_key: priv key bits set: 134/256
debug2: bits set: 564/1024
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host 'ms-ssh-rp.dev.ja.net' is known and matches the RSA host key.
debug1: Found key in /home/DEV/adamb/.ssh/known_hosts:1
debug2: bits set: 511/1024
debug1: ssh_rsa_verify: signature correct
debug2: kex_derive_keys
debug2: set_newkeys: mode 1
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug2: set_newkeys: mode 0
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug2: key: /home/DEV/adamb/.ssh/identity ((nil))
debug2: key: /home/DEV/adamb/.ssh/id_rsa ((nil))
debug2: key: /home/DEV/adamb/.ssh/id_dsa ((nil))
debug2: key: /home/DEV/adamb/.ssh/id_ecdsa ((nil))
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Next authentication method: gssapi-keyex
debug1: No valid Key exchange context
debug2: we did not send a packet, disable method
debug1: Next authentication method: gssapi-with-mic
debug1: Unspecified GSS failure.  Minor code may provide more information
Server <email address hidden> not found in Kerberos database

debug1: Unspecified GSS failure.  Minor code may provide more information
Server <email address hidden> not found in Kerberos database

debug1: Unspecified GSS failure.  Minor code may provide more information


debug2: we sent a gssapi-with-mic packet, wait for reply
debug1: Unspecified GSS failure.  Minor code may provide more information
Server <email address hidden> not found in Kerberos database

debug2: we sent a gssapi-with-mic packet, wait for reply
debug1: Authentication succeeded (gssapi-with-mic).
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Requesting <email address hidden>
debug1: Entering interactive session.
debug2: callback start
debug2: client_session2_setup: id 0
debug2: channel 0: request pty-req confirm 1
debug1: Sending environment.
debug1: Sending env LANG = en_GB.UTF-8
debug2: channel 0: request env confirm 0
debug1: Sending env LC_CTYPE = en_GB.UTF-8
debug2: channel 0: request env confirm 0
debug2: channel 0: request shell confirm 1
debug2: fd 4 setting TCP_NODELAY
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0