Suppose you have a law that says that in order to build new multi-unit housing, a given percentage of the new units have to be affordable housing and have to be built within reasonable distance of a transit stop in the same town. Meanwhile you have a town with poor mass transit and very little land zoned for multi-unit housing, and none of it overlaps. In other words, you're both required to and prohibited from building new affordable housing units near a transit stop. Which means you also can't build the other units.