A section of gas pipe without nearby junctions or interruptions spontaneously blocked itself. I've tried saving/reloading, as well as deconstructing and reconstructing sections of pipe. It seems to be a surprisingly robust bug. This pipe was flowing just fine when I first put it together, but while I was off doing other things, it blocked, and I came back to this. The attached screenshot isn't entirely ideal since you can't see the pipe inputs and outputs in screenshot mode, but the point should be clear. This isn't an issue of an incorrect build. 2 gas pumps feed into a T intersection, whose output goes through a VERY long pipe to a gas vent. It's that simple. It was also working, and then stopped working, without anything connected to that section of pipe changing.
I truly don't know how to reliably reproduce this bug, but I suspect the sheer length of the pipe is important, otherwise more people would be running into it. All I can really say is to build a MASSIVE liquid/gas heat exchanger, let it run for a while, maybe build some other stuff, save/reload, etc, and hope it happens.