Symptom:
Following sessmgr crash might be seen :

Assertion failure at sess/mme/mme-app/app/mme_app_util.c:16766
  Function: mme_app_relay_sgsn_dns_rsp_cb()
  Expression: (((mme_inst) != ((void *)0)) && ((mme_inst)->mai_magic == (('M'<<24)|('A'<<16)|('P'<<8)|('P'))))
  Proclet: sessmgr (f=87000,i=93)
  Process: card=16 cpu=0 arch=X pid=8060 cpu=~5% argv0=sessmgr
  Crash time: 2014-Aug-27+00:17:12 UTC
  Recent errno: 11 Resource temporarily unavailable
  Stack (2968@0xffffd000):
    [f7e285a1/X] libc.so.6/kill() sp=0xffffd078
    [0aa886a3/X] sn_assert() sp=0xffffd0a8
    [07799e3d/X] mme_app_relay_sgsn_dns_rsp_cb() sp=0xffffd138
    [0894eabc/X] snaptr_procedure_complete() sp=0xffffd498
    [0894ad16/X] async_return_cbk() sp=0xffffd4c8
    [051f9f0a/X] tlib_serv() sp=0xffffd508
    [051f9c2b/X] tlib_tmr_tick() sp=0xffffd538
    [046dab6f/X] snx_timer_service() sp=0xffffd558
    [046d950f/X] snx_fast_service() sp=0xffffd588
    [05cb8aa4/X] sessmgr_snx_tick_callback() sp=0xffffd5b8
    [0ab184b6/X] sn_loop_run() sp=0xffffdb48
    [0a902164/X] main() sp=0xffffdb88

Conditions:
MME running 15.0(55348)
MME queries DNS for an SGSN, then the MME service is removed or deactivated before the response is received.