So we were roofing today (finally!! the rain has been keeping us indoors)... anyway.. we were on a roof when all of a sudden, I look over and see people on my level.  The neighbor had called back his roofers to fix a leak.  I could see that it was a new roof (the houses were really close).  They had no idea where the leak was coming from.  One of them explained to me, "It's like a leak coming from a nail or something".  Really, clown!? How about the fact that you didn't check your step flashing around the chimney or even bother to put a starter underneath the first shingle over the back pan (starters are upside down shingles that make up for the keyhole of the properly oriented shingle)! This is all that was very obvious to me from a second's glance.  I never got on the neighbour's roof, but I'm sure there was a handful of other problems.  How do I know?  Because when someone who is unqualified to even stand on a roof can't find any sort of solution, they tar the shit out of everything.  Pardon my language but they really, really tarred the shit out of it.  It was dripping with black all over the chimney flashing and all over that first shingle that was missing its starter.  I've only seen something like it on a very old roof that needed to be replaced years before.  Ya.  I'm sure that homeowner got his money's worth.  

Here is another issue: the buckling of the metal (left photo), especially if the seam of the valley is not a perfectly straight line. The closed valley is much more attractive. And the most important point...