Incorrect ring gaps? KB Hypereutectic pistons, for example, require larger clearances than cast or forged, IIRC.

I have heard of a 383 sb chevy engine running for miles with a spun main bearing. Noisy, but still started and ran.

Spun, or misaligned, cam bearing will break the camshaft in half at that journal (brittle material). That happened to my dad's old 400M Ford.