Press the green flag and stare at the black cross. Notice there is no 'green circle' sprite. From where does it come, then?
"Troxler's fading or Troxler's effect is a phenomenon of visual perception. When one fixates a particular point, after about 20 seconds or so, a stimulus away from the fixation point, in peripheral vision, will fade away and disappear. The effect is enhanced if the stimulus is small, is of low contrast or equiluminant, or is blurred. The effect is enhanced the further the stimulus is away from the fixation point."
Retrieved from Wikipedia Nov. 1, 2008: http://en.wikipedia.org/wiki/Troxler%27s_fading