I have a kerberos ticket, I have pam_krb5 configured, and yet, I can't login through kerberos.

This works:
smbclient -U user -W G -L //foo.com/bar

This doesn't:
smbclient -U user -W G -L //foo.com/bar -k

This is with -d3

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lp_load_ex: refreshing parameters
Initialising global parameters
rlimit_max: rlimit_max (1024) below minimum Windows limit (16384)
params.c:pm_process() - Processing configuration file "/etc/samba/smb.conf"
Processing section "[global]"
added interface eth0 ip=fe80::216:41ff:fee3:8ae0%eth0 bcast=fe80::ffff:ffff:ffff:ffff%eth0 netmask=ffff:ffff:ffff:ffff::
added interface eth0 ip=172.21.41.66 bcast=172.21.41.255 netmask=255.255.254.0
Client started (version 3.5.4-63.fc13).
tdb(unnamed): tdb_open_ex: could not open file /var/lib/samba/gencache.tdb: Permission denied
resolve_lmhosts: Attempting lmhosts lookup for name foo.com<0x20>
resolve_wins: Attempting wins lookup for name foo.com<0x20>
resolve_wins: WINS server resolution selected and no WINS servers listed.
resolve_hosts: Attempting host lookup for name foo.com<0x20>
Connecting to 10.x.x.x at port 445
Doing spnego session setup (blob length=124)
got OID=1.2.840.48018.1.2.2
got OID=1.2.840.113554.1.2.2
got OID=1.2.840.113554.1.2.2.3
got OID=1.3.6.1.4.1.311.2.2.10
got principal=not_defined_in_RFC4178@please_ignore
cli_session_setup_spnego: got a bad server principal, trying to guess ...
Got challenge flags:
Got NTLMSSP neg_flags=0x62898215
NTLMSSP: Set final flags:
Got NTLMSSP neg_flags=0x60088215
NTLMSSP Sign/Seal - Initialising with flags:
Got NTLMSSP neg_flags=0x60088215
SPNEGO login failed: Logon failure
session setup failed: NT_STATUS_LOGON_FAILURE
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