Allthough my name is John, I was never called Jack. Never, until one of my son's coaches kept addressing me as Jack. He eventually had every parent on the team calling me that, and it became a family joke to call me "Jack".
After a commando style visit to WDW a few years ago (never again), I was doing research on these boards when the family began to call me Jack Disney.
They were even saying it when they were serious. One day on Main Street at the MK, my daughter asked me, "Hey Jack Disney, did you know about this new store?". The people around us were giving me curious looks