Actually, a fully retracted cam will not "cam" since the lobes have no way to properly move outward to distribute the force of a fall. They can either get stuck as mentioned, or slide outward / pop when force is applied to it. If the crack has a wider opening it might cam before exiting the crack, but the force of a fall can be enough to pull the cam outward faster than the spring can force the lobes to cam.
Read the manual that comes with the cam instead of tossing it aside!
Kevin, by that reasoning, no cams would work, ever. The springs have nothing to do with camming, the coefficient of static friction between rock and metal are what determines whether or not camming can occur. Otherwise, e.g. tricams would not work.