freebsd stable - Dec 2009 - panic: vm_page_alloc: cache page ... is missing from the free queue

This is 8-stable, amd64.
The system paniced under moderately heavy load after 3 days of continuous
package
building.
Unfortunately no dump.

panic: vm_page_alloc: cache page 0xffffff00cabc6180 is missing from the free
queue
cpuid = 1
KDB: stack backtrace:
db_trace_self_wrapper() at 0xffffffff801ad7f5 = db_trace_self_wrapper+0x2a
kdb_backtrace() at 0xffffffff8037d3c4 = kdb_backtrace+0x32
panic() at 0xffffffff8034dd2d = panic+0x1b0
vm_page_alloc() at 0xffffffff804d22df = vm_page_alloc+0x18f
vm_fault() at 0xffffffff804c2385 = vm_fault+0x7ae
trap_pfault() at 0xffffffff804fe017 = trap_pfault+0x181
trap() at 0xffffffff804feae1 = trap+0x29c
calltrap() at 0xffffffff804e5387 = calltrap+0x8
--- trap 0xc, rip = 0x8024ed032, rsp = 0x7fffffffdd60, rbp = 0x804e22100 ---

Any ideas what this could be? How to investigate further?
Is it likely to be a hardware issue?

-- 
Andriy Gapon