The first of these is catching the ball flat-footed. While you can have an active post player who moves well to get into position, I believe that a player who catches the ball with their heels on the ground is a poorer catcher. Catching flat-footed also causes a player to generally have static but not dynamic balance. Any undesired movement out of a static balance position when adjusting to catch a ball will cause the receive to lose their position of strength. Another common problem is a post player who exhibits stiff legs and arms.
Muscle rigidity makes it difficult to decelerate the ball and absorb extra energy in the ball.
Muscle rigidity in the legs means the lack of lower body shock absorbers results in a defender being able to exert force to displace the offensive post from a given position. In both cases rigidity makes it more difficult to catch a pass in the post. A third error often made in post catchers is when they call for the ball the finger-tips are pointed out. This results in the elbows being pointed slightly inward in an awkward position to flex and decelerate the ball from an incoming pass. I believe it is very important to have finger tips pointed more upward than outward.