Since all kinds of standard perceptual misjudgments -- say, mistakenly thinking that a facade in Fake Barn Country is a real barn -- are not really illusions, it follows that the contents of experience are rather more "thin" than sometimes supposed.* They involve basic things like colour, shape and length, but not such concepts as "barns" or the like. Your experience does not represent that there is a barn in front of you. It just represents the more basic visual qualities, and it gets all them quite right. The error comes later, when on the basis of this experience, you infer that there is a barn in front of you.
* = Byrne hence argues that the contents of experience have little to offer epistemologists. It's an interesting suggestion, but I won't discuss it further here.
Compare this to the Muller-Lyer illusion, where your experience (mis)represents that the two lines differ in length. The error is in the experience itself, rather than any further judgment. Indeed, the illusion persists even when you know the truth. Perceptual representations are hence informationally encapsulated from our reflective judgments or beliefs. The latter fail to correct the former. This might provide another argument for the "thinness" of perceptual content, since you probably will not continue to see the facade "as a barn" after learning the truth. Correcting the belief is enough. There's no enduring perceptual misrepresentation here.
These claims spell trouble for all sorts of proposed "thick" perceptions. Consider those who think that we "see" mental properties (e.g. another's happiness), or moral properties, etc. The illusion test flatly contradicts such proposals, since there is clearly no illusion involved in the relevant misjudgments (and nor do they persist after belief-updating). They go beyond the basic visual qualities presented in experience.