Hello,
   I recently set up samba to use kerberos authentication, but did not yet have a keytab in the filesystem.  Samba would panic and shutdown uncleanly when it tried to access the missing keytab.
   Placing the keytab on the filesystem cleared up the problem.
   Output of backtrace and testparm below:

Thanks!

# testparm
Load smb config files from /etc/samba/smb.conf
Processing section "[dfs]"
Loaded services file OK.
Server role: ROLE_DOMAIN_MEMBER
Press enter to see a dump of your service definitions

[global]
        workgroup = PHYSICS
        realm = PHYSICS.WISC.EDU
        server string = %h server
        security = ADS
        map to guest = Bad User
        kerberos method = secrets and keytab
        syslog = 0
        max log size = 100000
        client ldap sasl wrapping = sign
        dns proxy = No
        panic action = /usr/share/samba/panic-action %d
        idmap config * : backend = tdb

[dfs]
        path = /srv/smb
        msdfs root = Yes


[2016/04/06 16:42:59.330472,  1, pid=14365, effective(0, 0), real(0, 0)] ../source3/librpc/crypto/gse_krb5.c:416(fill_mem_keytab_from_system_keytab)
  ../source3/librpc/crypto/gse_krb5.c:416: krb5_kt_start_seq_get failed (No such file or directory)
[2016/04/06 16:42:59.330597,  0, pid=14365, effective(0, 0), real(0, 0)] ../lib/util/fault.c:72(fault_report)
  ===============================================================
[2016/04/06 16:42:59.330656,  0, pid=14365, effective(0, 0), real(0, 0)] ../lib/util/fault.c:73(fault_report)
  INTERNAL ERROR: Signal 11 in pid 14365 (4.1.17-Debian)
  Please read the Trouble-Shooting section of the Samba HOWTO
[2016/04/06 16:42:59.330737,  0, pid=14365, effective(0, 0), real(0, 0)] ../lib/util/fault.c:75(fault_report)
  ===============================================================
[2016/04/06 16:42:59.330795,  0, pid=14365, effective(0, 0), real(0, 0)] ../source3/lib/util.c:785(smb_panic_s3)
  PANIC (pid 14365): internal error
[2016/04/06 16:42:59.332309,  0, pid=14365, effective(0, 0), real(0, 0)] ../source3/lib/util.c:896(log_stack_trace)
  BACKTRACE: 33 stack frames:
   #0 /usr/lib/x86_64-linux-gnu/libsmbconf.so.0(log_stack_trace+0x1a) [0x7f2dd0a9d2ca]
   #1 /usr/lib/x86_64-linux-gnu/libsmbconf.so.0(smb_panic_s3+0x20) [0x7f2dd0a9d3b0]
   #2 /usr/lib/x86_64-linux-gnu/libsamba-util.so.0(smb_panic+0x2f) [0x7f2dd224ecaf]
   #3 /usr/lib/x86_64-linux-gnu/libsamba-util.so.0(+0x1cecf) [0x7f2dd224eecf]
   #4 /lib/x86_64-linux-gnu/libpthread.so.0(+0xf8d0) [0x7f2dd24788d0]
   #5 /usr/lib/x86_64-linux-gnu/libkrb5.so.26(krb5_storage_free+0x1) [0x7f2dc6c432a1]
   #6 /usr/lib/x86_64-linux-gnu/libkrb5.so.26(+0x47665) [0x7f2dc6c28665]
   #7 /usr/lib/x86_64-linux-gnu/samba/libgse.so.0(+0x8e06) [0x7f2dcbf35e06]
   #8 /usr/lib/x86_64-linux-gnu/samba/libgse.so.0(gse_krb5_get_server_keytab+0xeb) [0x7f2dcbf3632b]
   #9 /usr/lib/x86_64-linux-gnu/samba/libgse.so.0(+0xb03a) [0x7f2dcbf3803a]
   #10 /usr/lib/x86_64-linux-gnu/libgensec.so.0(gensec_start_mech+0x72) [0x7f2dcbd15ab2]
   #11 /usr/lib/x86_64-linux-gnu/libgensec.so.0(gensec_start_mech_by_ops+0xc) [0x7f2dcbd15bdc]
   #12 /usr/lib/x86_64-linux-gnu/libgensec.so.0(+0x8d62) [0x7f2dcbd12d62]
   #13 /usr/lib/x86_64-linux-gnu/libgensec.so.0(+0x9586) [0x7f2dcbd13586]
   #14 /usr/lib/x86_64-linux-gnu/libgensec.so.0(+0xa1f1) [0x7f2dcbd141f1]
   #15 /usr/lib/x86_64-linux-gnu/libgensec.so.0(gensec_update+0xa) [0x7f2dcbd1498a]
   #16 /usr/lib/x86_64-linux-gnu/samba/libsmbd_base.so.0(negprot_spnego+0x92) [0x7f2dd1dda492]
   #17 /usr/lib/x86_64-linux-gnu/samba/libsmbd_base.so.0(+0xcd9dc) [0x7f2dd1dda9dc]
   #18 /usr/lib/x86_64-linux-gnu/samba/libsmbd_base.so.0(reply_negprot+0x4aa) [0x7f2dd1ddb18a]
   #19 /usr/lib/x86_64-linux-gnu/samba/libsmbd_base.so.0(+0x123264) [0x7f2dd1e30264]
   #20 /usr/lib/x86_64-linux-gnu/samba/libsmbd_base.so.0(+0x12443f) [0x7f2dd1e3143f]
   #21 /usr/lib/x86_64-linux-gnu/samba/libsmbd_base.so.0(+0x124a29) [0x7f2dd1e31a29]
   #22 /usr/lib/x86_64-linux-gnu/libsmbconf.so.0(run_events_poll+0x171) [0x7f2dd0abbda1]
   #23 /usr/lib/x86_64-linux-gnu/libsmbconf.so.0(+0x44017) [0x7f2dd0abc017]
   #24 /usr/lib/x86_64-linux-gnu/libtevent.so.0(_tevent_loop_once+0x8d) [0x7f2dcf465f9d]
   #25 /usr/lib/x86_64-linux-gnu/samba/libsmbd_base.so.0(smbd_process+0xdb4) [0x7f2dd1e32fe4]
   #26 /usr/sbin/smbd(+0xa1d0) [0x7f2dd28b31d0]
   #27 /usr/lib/x86_64-linux-gnu/libsmbconf.so.0(run_events_poll+0x171) [0x7f2dd0abbda1]
   #28 /usr/lib/x86_64-linux-gnu/libsmbconf.so.0(+0x44017) [0x7f2dd0abc017]
   #29 /usr/lib/x86_64-linux-gnu/libtevent.so.0(_tevent_loop_once+0x8d) [0x7f2dcf465f9d]
  #30 /usr/sbin/smbd(main+0x1589) [0x7f2dd28aff19]
   #31 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x7f2dcf0d8b45]
   #32 /usr/sbin/smbd(+0x713b) [0x7f2dd28b013b]
[2016/04/06 16:42:59.333471,  0, pid=14365, effective(0, 0), real(0, 0)] ../source3/lib/util.c:797(smb_panic_s3)
  smb_panic(): calling panic action [/usr/share/samba/panic-action 14365]
[2016/04/06 16:42:59.448266,  0, pid=14365, effective(0, 0), real(0, 0)] ../source3/lib/util.c:805(smb_panic_s3)
  smb_panic(): action returned status 0
[2016/04/06 16:42:59.448542,  0, pid=14365, effective(0, 0), real(0, 0)] ../source3/lib/dumpcore.c:317(dump_core)
  dumping core in /var/log/samba/cores/smbd
[2016/04/06 16:42:59.464587,  3, pid=14357, effective(0, 0), real(0, 0)] ../source3/smbd/server.c:429(remove_child_pid)
  ../source3/smbd/server.c:429 Unclean shutdown of pid 14365