It always bugs me when optical illusions are brought up as instances of failure of the our perception system. These contrived conditions are really the 'exceptions that prove the rule:' our visual system has powerful inference capability running below the level of conscious perception, and most of the time it works so well we can't fathom the distinction between the model we infer and the sense data that the system works from. For instance, we perceive one scene with depth, when what we start with is two scenes with slight discrepancies.

Somewhat tangentially, I recall a psych paper where the researchers found that people perceive images in mirrors to be located behind the surface of the mirror. The researchers apparently thought that the image was at the surface of the mirror (ie, they misunderstood basic optics), and concluded that they'd discovered an optical illusion.