The biggest mistake a practitioner can do is to try to interpret the forms or even Chi Sau into applications.

Most of the movements are not directly applicable and are meant to guide the practitioner in developing a particular skill, like simultaneous arm movements, maintaining the vertical axes, and correcting errors or the development of the punch. One could say that Wing Chun is a method to develop the main weapon, your punch, and neglecting it would turn Wing Chun into the worst martial art in existence.
P.B.