A common "mistake" in Marxian theory is trying to arrive at what Marx "really thought" at various points in his life and/or in his many writings. What Marx meant is relevant certainly, and should be studied, if you are a historian of Marxian thought.
Marxian theories (like all overdetermined aspects of existence) evolve through time, and the writings of Marx himself should be viewed as a starting point, not dogma. Arguing over what Marx "meant" is academic masturbation not economic theory. These types of arguments are as wasteful of our time as trying to directly transform prices into values.