Some problems with KDB/scheduler?

I've been having some problems with KDB this evening; right now what I'm
bumping into appears to be a problem with interrupts and KDB/DDB.  An
example below.

(debugger entered using sysctl debug.kdb.enter=1)

db> break vlan_start
db> cont
tiger-3# 
tiger-3# 
tiger-3# ifconfig vlan0 create
tiger-3# ifconfig vlan0 vlan 100 vlandev em0
tiger-3# ifconfig vlan0 inet 192.168.60.1
[thread 100167]
Breakpoint at   vlan_start:     pushl   %ebp
db> cont
[thread 100167]
Breakpoint at   vlan_start:     pushl   %ebp
db> cont
[thread 100167]
Breakpoint at   vlan_start:     pushl   %ebp
db> cont
tiger-3# [thread 100071]
Breakpoint at   vlan_start:     pushl   %ebp
db> cont

tiger-3# [thread 100071]
Breakpoint at   vlan_start:     pushl   %ebp
db> cont
KDB: stack backtrace:
mi_switch(6,c5457300,c5457450,c5457300,e7868c48) at mi_switch+0x2ce
maybe_preempt(c5457300,e7868c40,c06033ce,197,c080c288) at
maybe_preempt+0xf9
sched_add(c5457300) at sched_add+0x26a
ithread_schedule(c5455500,c5404a80,12,0,c554da00) at ithread_schedule+0x8a
intr_execute_handlers(c53fc5e0,e7868ca8,12,e7868cec,c0793e03) at
intr_execute_ha
ndlers+0x65
lapic_handle_intr(42) at lapic_handle_intr+0x23
Xapic_isr2() at Xapic_isr2+0x33
--- interrupt, eip = 0xc0a27b71, esp = 0xe7868cec, ebp = 0xe7868cec ---
acpi_cpu_c1(e7868d04,c0619321,c08d1520,1,fffffffe) at acpi_cpu_c1+0x5
acpi_cpu_idle(c05f8328,c5403600,e7868d34,c05f811e,0) at acpi_cpu_idle+0xe1
idle_proc(0,e7868d48,0,c05f8328,0) at idle_proc+0x65
fork_exit(c05f8328,0,e7868d48) at fork_exit+0x7e
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xe7868d7c, ebp = 0 ---
panic: mi_switch: did not reenter debugger
cpuid = 0
boot() called on cpu#0
Uptime: 1m26s