"panic: fxp_start_body: attempted use of a free mbuf!" in RELENG_5

Top of RELENG_5 on an smp machine:

panic: fxp_start_body: attempted use of a free mbuf!
cpuid = 1
KDB: enter: panic
[thread 100042]
Stopped at      kdb_enter+0x30: leave
db> trace
kdb_enter(c06de69a,1,c06d47c5,ebbd2b14,c34cd320) at kdb_enter+0x30
panic(c06d47c5,c06c1030,4,c0499300,c3539000) at panic+0x14e
fxp_start_body(c3539000,0,c06d5639,4ca,c3539000) at fxp_start_body+0x462
fxp_start(c3539000,122,0,c3539000) at fxp_start+0x3f
if_start(c3539000,0,c06e7822,179,ffff8002) at if_start+0x99
ether_output_frame(c3539000,c38b5300,6,c4106b00,ebbd2bc8) at ether_output_frame+0x218
ether_output(c3539000,c38b5300,c4106b00,c95ec210,c04fe48d) at ether_output+0x43e
in_arpinput(c38b8700,122,c38b8700,c07656b8,c38b8700) at in_arpinput+0x686
arpintr(c38b8700,0,c06e7c88,e5,c07656b8) at arpintr+0x104
netisr_processqueue(c07656b8,c0738ac0,1,c06dda24,c3462940) at netisr_processqueue+0x8e
swi_net(0,0,c06dc037,269,c0738ac0) at swi_net+0xe9
ithread_loop(c345d880,ebbd2d48,c06dbe2a,323,d552d9c9) at ithread_loop+0x172
fork_exit(c04f1210,c345d880,ebbd2d48) at fork_exit+0xc6
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xebbd2d7c, ebp = 0 ---

Kris