decoding a panic

[Jason Price]

Recently, my varnish boxes have been seeing panics.  I don't know why.
vmod redis, mhash, and digest are in play (with the worst suspect being
vmod_redis).

Last panic at: Fri, 21 Nov 2014 17:16:55
GMT
[92/197]
Assert error in VRT_SetHdr(), cache_vrt.c line 225:
  Condition((sp)->magic == 0x2c2f9c5a) not true.
thread = (cache-worker)
ident =
Linux,3.4.68-59.97.amzn1.x86_64,x86_64,-smalloc,-smalloc,-hcritbit,epoll
Backtrace:
  0x431095: /usr/sbin/varnishd() [0x431095]
  0x43938b: /usr/sbin/varnishd(VRT_SetHdr+0xab) [0x43938b]
  0x7f19ee9f2151: ./vcl.W6jYfrXC.so(+0x3151) [0x7f19ee9f2151]
  0x7f19ee9f2def: ./vcl.W6jYfrXC.so(+0x3def) [0x7f19ee9f2def]
  0x437f78: /usr/sbin/varnishd(VCL_recv_method+0x48) [0x437f78]
  0x41957b: /usr/sbin/varnishd(CNT_Session+0xf2b) [0x41957b]
  0x432d5a: /usr/sbin/varnishd() [0x432d5a]
  0x7f19f8930f18: /lib64/libpthread.so.0(+0x7f18) [0x7f19f8930f18]
  0x7f19f8666b9d: /lib64/libc.so.6(clone+0x6d) [0x7f19f8666b9d]
sp = 0x7f1951865008 {
  fd = 0, id = 0, xid = 0,
  client = ?.?.?.? ?,
  step = STP_WAIT,
  handling = deliver,
  restarts = 0, esi_level = 0
  flags =

Any pointers on where to start looking?
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