I’m not a plumber, but I’ve been involved in the renovation/ construction business for quite some time and I ran across this problem in a house that always sprang leaks on cold copper water feeds. Turns out there were 2 things which may have contributed to the problem.
#1. The 1/2″ copper pipe used was “M” copper which has a much thinner wall than the typical “L” copper that most plumbers will use for installations in this area (NorthEast USA). In each instance there always was a small amount of green corrosion around the pin hole.
#2. The mechanical ground for the electrical panel was attached to the main cold water feed to the house using the copper pipe as the grounding rod. An electrician was called in to assess the situation and he installed a new ground rod and re-routed the ground cable so it wasn’t attached the the plumbing system…which is the way it should have been done in the first place. This seemed to have solved the problem as there hasn’t been any pin-hole leaks in about 4 years. So, whether it was the grounding problem alone, or a combination of the 2 is strictly speculation.
Hope this info is of some help.