Some rocks have faults that may end up giving you trouble if you use them in a dry stone wall. They might have questionable qualities that hardly show up at first, but when conditions get a bit tough, or you rely on them too much, they start to disintegrate and really let their side down.
You will recognize these rocks because they are generally quite coarse and rough around the edges. They are usually a bit flakey too. They would rather break the wrong way, than actually commit to any structure you are trying to build. They have such a questionable make-up that they may crumble away, shear apart or 'cut off' who-knows-what, to spite their face. There is no place for these kind of rocks in your wall.