This is an old, almost boilerplate issue in metaphysics––the "real distinction" between essence and existence (RDEE)––but there is one facet of the issue I'd like to bring up here based on my reading David Oderberg's Real Essentialism, at least for my own benefit, if not for the greater good of metaphysicians everywhere.
Briefly, RDEE entails that no entity includes its own existence in its essence (with the exception of God, though that is an issue I'll bracket here)*. In other words, to paraphrase Aristotle (Post. An. II, 7, 10), we can know what a thing is without knowing that it is (with the exception of indexicals and conscious experience, discussion of which I shall also bracket for now)**. In addition, we can know that some-thing exists without really knowing what it is (e.g. Bigfoot, UFOs, an approaching figure, the Wii game system, etc). So there is a bidirectional independence between essence and existence, form and substance. As Aristotle says:
"He who knows what human-- or any other-- nature is, must know also that man exists; for no one knows the nature of what does not exist-- one can know the meaning of the phrase or name 'goat-stag' but not what the essential nature of a goat-stag is. But further, if definition can prove what is the essential nature of a thing, can it also prove that it exists? ... [W]hat human nature is and the fact that man exists are not the same thing.... [It] is by demonstration that the being of everything must be proved--unless indeed to be were its essence; and, since being is not a genus, it is not the essence of anything. Hence the being of anything as fact is matter for demonstration…."
Hence, while Aristotle grants the role of pure, second-order reflection (imagination and notional extrapolation), he sharply divides a thing's supposed nature from its demonstrated existence.
Now, Oderberg addresses the objection that to assert RDEE "is to treat existence as some sort of characteristic of contingent things––a kind of metaphysical 'add-on' to essence" (p. 124). He rebuts, however, that this confusion is due to the paucity of vocabulary in contemporary metaphysics. He grants that "[e]xistence is indeed something that is true of existing things" and that "[a]ll existence ... requires that something be actualized", but denies that this makes existence a property, "since properties flow from the essence of a thing and, given what I have already said, existence does not flow from the essence of anything" (p. 124). Neither is existence an accident, Oderberg continues, "since accidents inhere in and modify already existing substances. Hence, to that extent, we can agree with the broadly Kantian argument against the Ontological Argument, to the effect that 'existence is not a real predicate'" (p. 124).
Even so, Oderberg argues, this should not lead us to the regnant Fregean view that existence is a second-order property of concepts, since "[e]xistence is something true of things that exist, not of our concepts of them" (p. 124). Even construed as properties of the world at large, the existence of things themselves is a fact about those things themselves, "and this is logically prior to its being a fact about the world."
It was at this point that my insight tried to strike me. I wondered where we should draw the line between the fact of x's existence and the fact of the world's actualization of x as a substance. As discussed above, we can't suppose any essence we can define could for that very reason be true in our world. Maybe there are far fewer essences--really possible forms--than we typically believe. Perhaps numerous essences are really impossible, though they are conceivable. Saul Kripke argues as much about unicorns (cf. Naming and Necessity [Harvard, 1980], pp. 23-24, 156-157), to wit, that given what we know about biological taxonomy, genetics, and natural causal history, unicorns are not really imaginable (recall Aristotle's reference to a stag-goat). All we can manage in terms of knowing the essence of unicorns are vague notions of horned horses. Beyond that, there is no coherent, compelling sense in which unicorns could be anything more than mutated horses. In any event, the upshot is that there is a two-way connection between facts about x and facts about the world in which x exists. What is the connection?
Let's go back to your invention above, the flying bed. You may be able to wax about its features, may have dozens of sketches and schematic blueprints, and may even have functional scale models, but, until it's made, you can't include in the blueprints the property "and it exists." Moreover, it may just be the case that, when the dream bed (which we'll call Bedd) is produced (which we'll call Bedder), unforeseen facts about the world impinge on the formal purity of your design (Bedd). This is a common problem in engineering, especially commercial engineering, where customer satisfaction can be as weighty as gravity. The tension here is between form and matter, design and development, shadow and act, Bedd and Bedder.
So let's imagine that features d, k, and s of Bedd have to be modified, or even removed, in order for it qua Bedder to function in the real world. At that point, facts about the world directly impede the fact of Bedd's existence, in which case, by modus tollens, the fact of the non-existence of Bedd is derived from a fact (or facts) about the world. This seems to refute Oderberg's point about existent-facts being logically prior to world-facts. Seems to. Oderberg's point is that the world "elk-izes" because elks exist in the first place. Kripke's point is that unicorns don't exist because the world doesn't unicornize. The juncture of these claims leads to believe that when we think we know a form (or an essence qua the abstracted state of a substantial form), we can only know it as far as it conforms to other substantial realities.
This may seem trivial, but my point is that what we know of a thing's form can actually be illuminated by its actualization in the world. If we find that features d, k, and s don't jibe with Bedder, then we realize what we thought we knew about Bedd (namely, its b-k-s characteristics) is actually no part of Bedd itself, even in its formal purity. What we thought we knew of Bedd as a really possible being in our world, is revealed in actualization not to have been produced by knowledge of the world. We know less about Bedd than we thought, because we know more about the world and Bedder than our grasp of Bedd could ever fathom. As James Ross puts it, "Material things overflow our conceptions", and this because "the de re necessities [for anything] spread out into the inaccessible" (Thought and World [University of Notre Dame Press, 2008], pp. 14-15).
Once we see why Bedder cannot b-k-s, we will just see why that it is so: Bedd's substantial, as opposed to purely abstract, form requires non-b-k-s. The "design constraints" put on Bedd arise from matter, but its actual coming-to-be depends on the potentiality--the ontic hospitality, as it were--of materiality qua the womb of the world. It may be true of the world that the world Beddizes only in virtue of the fact that Bedder exists (i.e. the existent-fact has logical priority over a state of affairs), but knowing what Beddizing really looks like requires that we know how the world actualizes Bedd. Strangely enough, then, while the existence of Bedder is not a part of its essence (viz. Bedd), the existence parameters of Bedder were facts about Bedd all along. Certain impossible facts about Bedd (i.e. its b-k-s features) betray our misconception of facts about the world from which we assumed we had derived it.
* [In a nutshell the distinction hinges on the fact that God's very Godness just is the essential inclusion of existence.]
** [In a nutshell these latter things are either derivative modifications of a prior substance or beings of reason (second intentions) which exist by reflection on prior objects.]