Funny you mention that - I had the same kind of thing just a couple nights ago. I aired up a section of new & old 1-1/4" piping, and it was leaking at a 17 year old solder joint on the sweat side of a sweat/NPT adaptor. I might have been a bit rough on that section when I was dismantling my old boiler (had to use an extension on the pipe wrench), but I had it well supported when I was doing that. I haven't re-checked it yet since I re-sweated it. I haven't had many leaks yet (knock on wood), the ones I have had so far were on solder joints.