Take a look at this. Focus on the “+” in the middle, and describe what you see.
Troxler Effect image
No, the purple dots don’t disappear, though that’s probably what you “see.” Worse, there’s no green dot. Your brain sees green when the purple disappears — and even when your brain refuses to let you know the purple dots are still there, it will tell you you see a green dot when the purple dots you can’t see, disappear.
So, is it so hard to understand that people might see weird things in the mirror, if they stare at their own faces for a while?
Cortical Hemming and Hawing has the full story, with a history of the Bloody Mary story. Go see.
Posted by Ed Darrell 
Millard Fillmore's Bathtub 