Galvanized Pipes Rusting From Within.

Here are two great examples of galvanized drain pipes rusting from within.  In the top photo, you can see how these become blocked over time.  The pipe looks ok on the outside, but inside it's a different story.  Eventually the rust reaches the outside of the pipe and you have pipes that look like the ones in the lower photo.  These leaks start as pinholes that often self seal with more rust.  anytime you see what looks like a rusty scab on a pipe it's an indication that the pipe is failing from within.  Another test is to knock it with something metal.  The more metal on metal sound the better.  If what you hear is nothing more than a dull sounding thud, then you know the pipe has got some serious corrosion inside.  

Depending on access, this sort of problem can be solved relatively easy with PVC piping and rubber couplings.