I got my DEBUG enabled kernel crashed with the attached test,
	first it breaks system to ddb and an attempt to continue
	ends up in ofw 'ok' prompt. Commenting out lines below helps.
	locore.s:4795
	#ifdef DEBUG
		tst	%g2
		tz	1		! tpc NULL? Panic
		tst	%i6
		tz	1		! %fp NULL? Panic
	#endif

That check is there to make sure the cpu is not trying to return from a
trap with either a NULL pc or stack pointer.  The machine is unlikely to
recover in that event, and this the trap lets you determine what the problem
is.

Eduardo