Just experience. Been there done that. Lost a freshly built 302 cause of it. Its what happens when you build an engine with your buddies hanging around and a case of beer. At least in my case anyhow. Mine sent a sliver of a cam lobe into the oil pump, broke the shaft and basically destroy'd the motor. I should have clued in earlier cause the igniton timing changed all on its own a few days before it let go. Big lesson learned on that build. I was about 18 at the time so its funny now. Not so funny for you. Good thing you caught it before something broke and took the engine out.
I had a roller cam walk too, but it was due to using a non roller cam retainer plate. The steel roller cam ate into the (apparently) iron retainer plate I mistakenly used in that one build. I now mark roller cam retainer plates when I pull them from roller engines to avoid making this mistake again. In any case, it didn't walk as far as Masori's did. I'm not sure if Ford used hardened iron retainer plates in the roller engines or cast these from steel, they're really impossible to tell one from another if you don't mark them as you pull em.
