1. Mistaken perception is one of the most common sources of unreality: we see, hear, smell something and take it to be something different than it really is; sometimes we even act on misperceptions, and then we have to correct not only our erroneous views, but also take responsibility for our misguided actions. In our everyday lives, of course, mistake and correction happen in quick sequence (for instance, when you think you see a person whom you know from a distance, and wave a greeting, merely to discover after a few quick steps towards them that it wasn't your acquaintance after all); the consequences are mostly negligible, or corrected without much effort. The basic pattern, however, is still the same as in Poe's story: we take in some sketchy information, interpret the situation wrongly, and then act in line with our false views. (The fault, dear reader, is of course not in our perception, but in ourselves; I've emphasized that point in an earlier post on "The Spectacles" and perception.)
Observations such as these suggest that we generally have a certain practical interest in keeping close to reality, which is why in our everyday lives we usually try to double-check whatever we perceive (or remember, learn by hearsay from others, hypothesize, or otherwise get out of sources which we know may mislead us on occasion). We seek small reality-checks in much of what we do, in order to navigate our surroundings without drifting too far into unreality, because we know from experience that we are likely to be more successful in everyday life if we go with the flow of what's really going on, and adjust our course if necessary. We have a constant habit of eliminating unreality from our views in order to succeed in our activities and reach practical goals, a habit I call reflection. (In some areas of philosophy, there's a much more narrow and technical use of the term 'reflection'. That's not the sense I have in mind here; reflection, in the way I've introduced it, has more to do with the common sense notion of taking a step back and a deep breath, and calmly checking things over before forming a view or taking action.) Reflection has the function of keeping us close to reality in what we think and do.
2. In an interesting passage in the Philebus, Plato starts out with a perception example not unlike the one I've used (and the one that is, greatly exaggerated, at the basis of Poe's story as well), and generalizes this to other forms of unreality. Most interestingly, he discusses forms of unreality connected to the past and future, and that's what I'd like to take a closer look at here.
In that passage, Socrates (Plato's lead character) sketches an account of what happens when we perceive something. For example, "it often happens that someone who cannot get a clear view because he is looking from a distance wants to make up his mind about what he sees"; when making up his mind, the perceiver would ask himself: "'What could that be that appears to stand near that rock under a tree?'". This way of describing it emphasizes the process character of what goes on: we take in the scene, and sometimes, when it is not clear and obvious what it is we perceive, we first have to decide what to make of how things seem to us. Only then it becomes a judgment (whether it is spoken out loud or remains implicit in what we feel and how we behave). And of course, judgments can be correct or incorrect; we might misjudge the situation, which happens all the time with perception: it can lead us astray. In the example, the man might correctly judge that it is a person what he sees; "he might also be mistaken and say that what he sees is a statue, the work of some herdsmen".
In both cases, the judgment can be neatly expressed as a sentence, as I just did at the end of the previous paragraph for both the true and the false version. It has, to put it in philosophical jargon, propositional content. However, Socrates makes it clear that this is not all there is to a particular act of perception. There is also all the sensual input itself, which is in this case visual input (but it could also be input from other senses, i.e. auditive, tactile, or olfactory). The content is not exhausted in the mere word-content. There are also the images that you see. If you'd write down the sentence and then text it to someone, you would have transferred only part of the content. (Maybe if you'd take a photo and send it along with the text, you'd have transferred more, or even most of it.) So, in addition to the propositional content (the 'word' content), there's also what we might call pictorial content.
Right from the beginning, Socrates stresses that his account also applies to what happens when we remember something. Again, memory can fail us, and we might remember something incorrectly. And once more, to our false memory, there is not only the propositional content of what we remember ("I remember having seen this street at daylight.") There are also the images, which we can revive in our mind's eye. And as we all know, vivid and even convincing-seeming as such images can be, they're extremely unreliable.
Having covered the present (in perception) and the past (in memory) in their function as "lead[ing] to judgment or the attempt to come to a definite judgment, as the case may be" (38b), Socrates finally extends this to an analogue in the future: in hopes. (It seems to me he should better have used a neutral term, such as 'projections' or 'expectations', since hopes normally are associated with positive expectations only. I presume this choice is because what interests Socrates is the pleasure that we take in them, and we wouldn't find that in negative expectations, thus the restriction. For a general account, however, hopes would be only one side of the medal, the other being fears and the like.) As with memories and perceptions, hopes (which Socrates identifies with "assertions in us", that is, presumably the sentence formulations of what we expect to happen to us in the future) are associated with images: "someone often envisages himself in the possession of an enormous amount of gold and of a lot of pleasures as a consequence. And in addition, he also sees, in this inner picture himself, that he is beside himself with delight."
The structural claim here is that if there is a kind of unreality we have to cope with in the present (in the form of false perception) and in the past (false memories), then there must also be something similar in the future (false hopes). This premise is emphatically confirmed by Socrates' interlocutor, which probably indicates it's an uncontroversial premise; at least Plato wants to take it as one for the purposes of this dialogue. (Compare also La. 198d for a similar structural claim about knowledge of present, past and future.)
When we're looking for unreality, then, be it in small and simple instances as in everyday life or even in elaborate illusions as in Poe's story, we must check for all areas: not just what's directly before our eyes, but also what's before us in time, and what has been before. A phenomenology of the unreal will thus have to cover the future and the past in addition to the present.
 38c–39e. All quotes (unless otherwise indicated) are from that passage, taken from the Hackett edition: Plato, Philebus. Translated, with introduction, by Dorothea Frede. Indianapolis: Hackett 1993.
 The context is the notorious discussion of 'false pleasures', where Socrates argues that pleasures can be literally false, just in the same sense in which opinions can be false. He lists four different classes of false pleasures, and our passage here is taken from the exposition of the first of these classes. For my purposes, the connections to the theory of pleasure aren't relevant, and I've systematically left them out.
 Plato's argument is sketchy, and it proceeds by analogy; of course, much more would be needed for a full-blown account of perceptual content, and from a modern point of view, several serious questions would have to be raised. I won't discuss the question whether the account Socrates gives is sufficient for the purposes of his own argument; for me, the important aspect is rather the parallel with other forms of unreality, such as false memories and hopes, which we'll get to in a moment.
 Socrates goes on to claim that what causes us pleasure is within the pictorial content, and thus if the pictorial content is false, i.e. an instance of unreality, then the pleasures are 'false' in his sense. As noted above, I'm not interested in pleasure here. If you want to look further into this debated notion, a good place to start is Dorothea Frede's introduction (and its bibliography) to the Hackett edition. See note  above for the bibliographical reference.