A student asserts, “If an act is prohibited, every legal consequence of that act must always be void.” This is too broad. The book distinguishes prohibition attached to the act itself from prohibition connected to an external attribute; legal effects must be examined separately.
Correct method: identify what precisely is prohibited, then consider the relevant rules for validity or other consequences. Equally, the fact that a prohibited act may have some legal effect cannot be used to declare it morally permitted.