Last night under heavy load my workstation rebooted a few times for no
apparant reason. I built a kernel with DDB and DIAGNOSTICS, and sure
enough, while compiling the qt2 package from pkgsrc it died and I caught
the error (copied by hand):

uvm_fault(0xcb1e2bd0, 0x0, 0, 3) -> 1
kernel: page fault trap, code=0
Stopped in pid 22894 (cc1plus) at      statclock+0x79: addb    %al,    0(%eax)
db> t
statclock(...) at statclock+0x79
hardclock(...) at hardclock+0xb2
clockintr(...) at clockintr+0xb
Xintr0() at Xintr0+0c6b
--- interrupt ---
 0x80ca49c:
db>