Years ago, I was swapping out a 225 for a 360 in my 72 Dart. I bought a running 79 Doda that had been hit in the left rear hard enough to S shape the leaf spring. Once I got it out of the Cordoba I put it on an engine stand and pulled the pan to look at the bearings. Based on the date codes and other pointers I was pretty sure no one had ever been in there before.

I found a nut in the crankcase. Specifically a con rod nut. That engine had 17 con rods nuts inside it, 16 installed, one loose. I can only assume that during the build someone dropped a con rod nut and just grabbed another rather than find the loose one.