Illusion exists in many places and many forms, but the most common is within the perceptions of the human mind. Perfection, for instance, is a lie in the form of absolutes. Only within the perception or imagination of an individual can there be perfection at all. That which you name as perfection is titled as flawed by another. Simple conjecture is relied upon by our minds. We assume and run with that assumption in place of the missing facts.

Granted, this is not a horrible thing on its own, but only a fact of humanity in general. The negative effects occur once ignorance, arrogance, or perhaps fear are combined in some manner. In religion the flaw is apparent by the people who rely on faith and yet condemn others; claiming knowledge of objective truth while only being capable of attaining subjective knowledge.

Beyond that example, and within it as well, is the illusion of free will. Much religious belief relies on the existence of free will. Once broken into identifiable parts, free will fails against even the most basic definitions of determinism and chaos theory. Everything both affects and has effects, though they are not perhaps always easily noticable. Even the most disorganized seeming of items or instances in existance are precisely ordered.