FreeBSD 6.1-STABLE: Unexplained power off

[Christian Walther]

This is just a wild, uneducated guess, because I'm not a long FreeBSD
user, but from my point of view this error could really be related to
ACPI/APM, as already has been suggested.
Maybe the machine is trying to go to suspend, but fails while doing
so, which in the end would mean that it can't recover from the
suspend, but has to reboot completely, resulting in dirty file
systems. It wouldn't reach the suspend state correctly, which could
leave everything depending on ACPI/APM in a undefined state, including
the hardware. This would explain why the machine has to be turned off
properly by pressing toe power button for such a long time.

I'd try to use the machine without ACPI/APM enabled. If possible,
compile a new kernel without it being enabled. This might not be
possible because you're on a SMP-system, thou, but you might want to
check your configuration files for suspend or hibernation -- and turn
them of.
With ACPI/APM turned on, leave the machine idle for some time and see
if it shows the same behaviour. When it shuts down cleanly it's likely
that suspend/hibernation fails due to the high load introduced by the
build process.

I've seen and experienced similar problems on other platforms, such as
OS X and (sorry) Linux.