Our brains are wired to receive and interpret input, but we are usually oblivious to all this visual processing going on behind the scenes. One of the most impacting optical illusions is called The Spiral Aftereffect. What makes it impacting is not what it does to you while you're looking at it, but what it does to you after you look away, i.e., when you stop looking at it. Here are the instructions:
Stare at the center of the rotating spiral for about 20 seconds, then look elsewhere. You will notice that whatever you look at now appears swirling. (Don't worry, it won't last very long), this sensation will go away after a few seconds. [...] Gazing at the spiral for a longer time will increase the duration of the aftereffect, leveling out at ≈ 30 s. Holding the gaze steady at the center (don't over-concentrate, “let it hang”) also increases the effect. [Now click here.] 
Since many of our neuronal structures are, functionally speaking, pattern detecting modules, processing efficiency is increased when the pattern is presumed to be continually present. This presumption fades when there is no continual feedback from perception. Of course, that's good, since one would would otherwise be forever locked into a illusory pattern of perception interpretation. The brain must set some temporal window on pattern presumption.
In psychology labs, people don special mirror masks that up-down invert or left-right symmetrically switch the visual field. At first, this make them sick. Later, the brain adjusts and eventually they don't even notice that their world has been turned upside down--until they take them off. At that point, the the motion sickness symptoms reoccur until the brain readjusts itself.
Of course, one does not have to wear a mirror to have misperceptions, since people don't properly interpret what they see in a mirror anyway.
[image] “Floating Motion” from Pinna & Spillmann (2002) Just by exploratory eye movements over the image the centre square “decouples”, so the background seems to move, while the central square remains in place, and seems to float on top. Linked from 72 Optical Illusions & Visual Phenomena Website (Accessed 7/30/2007)
"Spiral aftereffect" 72 Optical Illusions & Visual Phenomena(Accessed 7/30/2007)
 "Humans Do Not Understand Mirror Reflections, Say Researchers" PHYSORG.COM (Accessed 7/30/2007))