Headers are one of those things that every one of you uses in massive quantities, for everything. And they are made of 2 elements: metal and plastic. What can possibly be or go wrong with a plain simple header? This:
Yeah, a short. What?! Inside the header?! I found this a while ago on a returned Moteino that did not work after the header was soldered. The SPI bus was acting weird, I only discovered that after hooking up the scope. Continuity testing revealed a possible short between D12-13, but where? The traces on the PCB were good, maybe a soldering connection hidden under the header base?
Ok so this only happened to me once, that I could detect and prove. But enough to cause a confusing frustration on the part of the user, time and money spent to return it, and the time spent to guess-debug what could possibly be wrong in an unthinkable place. I still need headers and will continue to carry them. But if something like this happens to you, check the headers!
Good lesson china, we thank you.