Aristotle sometimes claims that (i) the perception of special perceptibles by their proper sense is unerring. This claim is striking, since it might seem that we quite often misperceive things like colours, sounds and smells. Aristotle also claims that (ii) the perception of common perceptibles (e.g. shape, number, movement) is more prone to error than the perception of special perceptibles. This is puzzling in its own right, and also places constraints on the interpretation of (i). I argue that reading Alexander of Aphrodisias on perceptual error offers an understanding of Aristotle that can help us to make good sense of both of Aristotle’s claims.