Cheers Dave, ... as I thought but was just trying to guess why this might have occurred, I thought perhaps a different type of fastening from that expected in that location might have thrown the mechanic; the scenario in tdracer's reply sounds plausible to me.
As a practical man, this incident gives me a distinct feeling of "there but for the grace of God go I". However much I train myself not to be distracted, something unexpected can still cause trouble!