About 6 months ago we had our lead rising main replaced with
copper. Yesterday it developed a small leak from mid-way along
a section of pipe. On cutting it out I found the pipe showed
no signs of having been abraded on the outside; instead there
was a pinprick-sized hole, and inside the pipe there was a
small (3mm) cluster of green stuff (presumably, copper oxide).

I have never encountered this before. Would it be due to a
flaw in the pipe, or is there something (e.g. some other
metal) which can lodge on the inside of a pipe and rot it?