[Gaz64]

I'd be looking at all the reasons for the necessity of lifter rotation.

Lifter base radius, cam lobes ground with the taper to suit, lobe to lifter bore offset, lifters not "hanging up" or binding in the bores, excessive camshaft end float.

One lobe can fail on it's own, or it could be a sign of others about to follow.

Obviously once something like this happens, investigation generally starts before any further damage occurs.