I am a contributor to a very old project presently licensed under the GPL. Our project is a plugin for a closed-source program, but the GPL does not permit this kind of linking. The GPL FAQ states:
"If the program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single program, which must be treated as an extension of both the main program and the plug-ins. This means that combination of the GPL-covered plug-in with the non-free main program would violate the GPL. However, you can resolve that legal problem by adding an exception to your plug-in's license, giving permission to link it with the non-free main program."
This was interpreted to mean that WE were in violation of the GPL on our own code because the GPL does not permit dynamic linking between Free and non-Free code.
bmargulies pointed out we that aren't violating the GPL, since a violation occurs when someone combines the GPL library with the closed source component and then distributes the result. Therefore it's OK for us to consider linking to the closed-source host as OK and not pursue it, and no third party can feasibly take legal action against us for this.
This is great, but what if our project included in its source code GPLed source code from someone else's project? Can that project's owners decide that our interpretation of linking doesn't fly with them and take action against us?