Lenstore shares this wonderfully weird double optical illusion:
Follow the light grey spot around the circle for 30 seconds to 1 minute, and you will notice the other spot will eventually turn green. If you stare at the cross in the middle for 30 seconds to 1 minute, the spots around the circle will disappear.
The vanishing is spots is an example of the Troxler Effect, named for Swiss physician/philosopher Ignaz Paul Vital Troxler (1780-1866). From the Illusions Index:
In 1804, Troxler made the discovery that rigidly fixating one's gaze on some element in the visual field can cause surrounding stationary images to seem to slowly disappear or fade. They are replaced with an experience, the nature of which is determined by the background that the object is on. This is known as filling-in.
The Troxler effect illustrates the importance of saccades, the involuntary movements of the eye which occur even while one's gaze is apparently settled. If we could perfectly fixate on some point in our visual field by suppressing saccadic movement, a static scene would slowly fade from view after a few seconds due to the local neural adaptation of the rods, cones and ganglion cells in the retina. In brief, any constant light stimulus will cause an individual neuron to become desensitized to that stimulus, and hence reduce the strength of its signal to the brain.
