A specific illusion of consciousness
An illusion is defined as either an erroneous perception of reality or an erroneous concept or belief. Whichever of these two modes of illusion we may want to consider, illusion is understood as a mode of consciousness. In other words, all illusions are usually thought of as specific to consciousness!
However, there is no good reason to assume that all of the perceptions that a creature with a brain has are conscious. Indeed, our consciousness is routinely focused of a small part of our perceptions, and indeed often focused on the "perception" of the inner world of our own ideas, rather than on that of the outside world. Thus, it seems impossible to tell whether illusions are really specific to consciousness.
Acting without focusing our consciousness on what we are doing will more likely, and indeed very often, end up in mishap, if not disaster, demonstrating the presence of some error of perception or some error of belief, at some, presumably unconscious, level, and therefore the presence of some illusion. Consciousness might even be the instance of last resort to dispel illusions before any mishap could ensue.
But there is no hiding from the fact that some illusions are conscious. What is more difficult to say is whether any particular kind of illusion is specific to consciousness, since, by definition, we wouldn't be aware of our non-conscious illusions, if any.
absolute certainty would be an illusion of consciousness
Naive realism is the term used to identify our native belief, and indeed native absolute certainty, that our senses provide us with direct awareness of our environment as it really is. Indeed we mistake our perceptions themselves to be the world around us. That is, we have the illusion that our perceptions are the world around us.
Although philosophers all over the world and very early, at least in history properly so-called, have understood this illusion, we can't quite shake it off. In this sense, it seems by essence the illusion that inheres in consciousness.
As such, it is also perhaps the question that has been the most discussed ever by philosophers since the beginning of philosophy.
The more recent development of the debate about the possibility of knowledge of the material world should be seen as a mere extension of our discussion of naive realism.
The extension comes entirely with the drive to substantiate the possibility of our knowledge of the outside world. Given that we humans will always insist that we know at least some things in the outside world, many philosophers are currently trying to flesh out what such knowledge really consists of, and this even though past claims to knowledge are routinely falsified by our own current beliefs.