I all so appear to be getting this error. I have enabled 'debug level = 3' in my smb.conf so am waiting for another error to get some more information. It happens a random amount of times a day. It appears to happen when you connect to a share or try to open a file on the share. I have only experienced the problem on Windows 7 client but I don't access the share often from Ubuntu clients.

smb.conf - (with out comments)
-----------------------------------------------------------------------------------
[global]
   workgroup = WORKGROUP
   server string = %h server (Samba, Ubuntu)
   dns proxy = no
follow symlinks=yes
unix extensions = no
   debug level = 3
   log file = /var/log/samba/log.%m
   max log size = 1000
   syslog = 0
   panic action = /usr/share/samba/panic-action %d
   encrypt passwords = true
   passdb backend = tdbsam
   obey pam restrictions = yes
   unix password sync = yes
   passwd program = /usr/bin/passwd %u
   passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
   pam password change = yes
   map to guest = bad user
   usershare allow guests = yes
-----------------------------------------------------------------------------------

lsb_release -rd
-----------------------------------------------------------------------------------
Description:	Ubuntu 11.04
Release:	11.04
-----------------------------------------------------------------------------------

uname -a
-----------------------------------------------------------------------------------
Linux niall-desktop 2.6.38-11-generic #48-Ubuntu SMP Fri Jul 29 19:02:55 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux
-----------------------------------------------------------------------------------

apt-cache policy samba
-----------------------------------------------------------------------------------
samba:
  Installed: 2:3.5.8~dfsg-1ubuntu2.3
  Candidate: 2:3.5.8~dfsg-1ubuntu2.3
  Version table:
 *** 2:3.5.8~dfsg-1ubuntu2.3 0
        500 http://ucmirror.canterbury.ac.nz/ubuntu/ natty-security/main amd64 Packages
        100 /var/lib/dpkg/status
     2:3.5.8~dfsg-1ubuntu2 0
        500 http://ucmirror.canterbury.ac.nz/ubuntu/ natty/main amd64 Packages
-----------------------------------------------------------------------------------


-----------------------------------------------------------------------------------
[Thread debugging using libthread_db enabled]
0x00007fd99f6b01ae in waitpid () from /lib/x86_64-linux-gnu/libc.so.6
#0  0x00007fd99f6b01ae in waitpid () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007fd99f644d6e in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007fd9a2595c53 in smb_panic (why=<value optimized out>) at lib/util.c:1471
#3  0x00007fd9a25852e9 in fault_report (sig=11) at lib/fault.c:52
#4  sig_fault (sig=11) at lib/fault.c:75
#5  <signal handler called>
#6  0x00007fd99f6855d1 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#7  0x00007fd9a256c11e in rep_strlcpy (d=0x7fff506b6ff8 "", s=0x0, bufsize=256) at ../lib/replace/replace.c:69
#8  0x00007fd9a25a2434 in connections_fetch_entry (mem_ctx=0x0, conn=0x7fd9a3657150, name=0x0) at lib/conn_tdb.c:65
#9  0x00007fd9a2323676 in yield_connection (conn=0x7fd9a3657150, name=0x0) at smbd/connection.c:33
#10 0x00007fd9a23947f6 in close_cnum (conn=0x7fd9a3657150, vuid=100) at smbd/service.c:1256
#11 0x00007fd9a235016d in reply_tdis (req=0x7fd9a3650910) at smbd/reply.c:4912
#12 0x00007fd9a238fbb5 in switch_message (type=113 'q', req=0x7fd9a3650910, size=39) at smbd/process.c:1404
#13 0x00007fd9a238ff57 in construct_reply (inbuf=<value optimized out>, nread=39, unread_bytes=0, seqnum=0, encrypted=false, deferred_pcd=0x0, conn=<value optimized out>) at smbd/process.c:1436
#14 process_smb (inbuf=<value optimized out>, nread=39, unread_bytes=0, seqnum=0, encrypted=false, deferred_pcd=0x0, conn=<value optimized out>) at smbd/process.c:1509
#15 0x00007fd9a23903da in smbd_server_connection_read_handler (ev=<value optimized out>, fde=<value optimized out>, flags=<value optimized out>, private_data=<value optimized out>) at smbd/process.c:2126
#16 smbd_server_connection_handler (ev=<value optimized out>, fde=<value optimized out>, flags=<value optimized out>, private_data=<value optimized out>) at smbd/process.c:2141
#17 0x00007fd9a25a5303 in run_events (ev=0x7fd9a355e430, selrtn=1, read_fds=0x7fff506b7470, write_fds=0x7fff506b73f0) at lib/events.c:157
#18 0x00007fd9a2391936 in smbd_server_connection_loop_once () at smbd/process.c:844
#19 smbd_process () at smbd/process.c:2455
#20 0x00007fd9a289091e in smbd_accept_connection (ev=<value optimized out>, fde=<value optimized out>, flags=<value optimized out>, private_data=<value optimized out>) at smbd/server.c:438
#21 0x00007fd9a25a5303 in run_events (ev=0x7fd9a355e430, selrtn=1, read_fds=0x7fff506b7790, write_fds=0x7fff506b7710) at lib/events.c:157
#22 0x00007fd9a25a54f8 in s3_event_loop_once (ev=0x7fd9a355e430, location=<value optimized out>) at lib/events.c:220
#23 0x00007fd9a25a5f90 in _tevent_loop_once (ev=0x7fd9a355e430, location=0x7fd9a2a970c8 "smbd/server.c:726") at ../lib/tevent/tevent.c:497
#24 0x00007fd9a28915c3 in smbd_parent_loop (argc=<value optimized out>, argv=<value optimized out>) at smbd/server.c:726
#25 main (argc=<value optimized out>, argv=<value optimized out>) at smbd/server.c:1316
A debugging session is active.

       Inferior 1 [process 7823] will be detached.

Quit anyway? (y or n) [answered Y; input not from terminal]
-----------------------------------------------------------------------------------