I had a machine which kept rebooting spontaneously: no crash dump, no message, nothing; only a sudden reboot with nothing in the logs.
Adding INVARIANTS, WITNESS, SOCKBUF_DEBUG, DEBUG_VFS_LOCKS, DEBUG_MEMGUARD and DIAGNOSTIC did not produce any message.
I threw DDB into the picture, but it won't be brough up; only a plain reboot.
The culprit seems to be splash, as this behaviour stopped when I disabled it.
Steps to reproduce:
_ install 10.1 or 10.2 i386 (no installed port or running deamon needed);
_ put the following lines in /boot/loader.conf:
_ choose a 320x200x8 image for /boot/splash.bmp;
_ issue a "make kernel": it will rarely get to the end, most of the times rebooting within seconds.
Put 'blanktime="NO"' in /etc/rc.conf and kernel can be built again.
This happens on i386, but not on amd64.
Altough I have no 9.x or earlier box to try now, I've been using this config for several years and never experienced this problem.