On 13 Jun 2005 00:40:06 -0700, wrote:

instead there was a pinprick-sized

Pinholes are often caused by uncleaned flux residues. Any sign of that
on the outside?

and inside the pipe there was a
small (3mm) cluster of green stuff (presumably, copper oxide).

Copper oxide is either red or black, for cuprous or cupric oxides (I
forget which is which). However most other copper salts are blue or
green, so it could be almost anything.

I have never encountered this before.

Nor I.

Would it be due to a flaw in the pipe,

It would be a rare flaw that left a pinhole in copper pipe - they do
happen, but they normally cause splits. It _might_ be an inclusion, that
then gave rise to corrosion.

or is there something (e.g. some other
metal) which can lodge on the inside of a pipe and rot it?

This could certainly happen, but I'd be surprised that such a
contaminant could sit there without getting washed out.

Any chance that the hole came first, then the crystals formed around it?