In my case it was a spread out wire terminal on the oil pressure swith. It conveniently wiggled off the idiot light sender some undetermined amount of time before the oil plug wiggled free. Thus no warning light when the oil dumped out on the road, and the new engine didn't even know enough to get a bit rattly like an old one would. No signs at all until it just started squeaking.
It didn't throw a rod, but spun one of the rod bearings turning both the rod and the crank a nice shade of blue. Not a cheap lesson about checking everything twice and minding all the details, no matter how good a job you do on the 'big' things...