What to see
Below you see a rotating array of blue crosses and 3 yellow dots. Now fixate on the centre (watch the flashing green spot). Note that the yellow spots disappear once in a while: singly, in pairs or all three simultaneously. In reality, the 3 yellow spots are continuously present, honest!

What to do
You can use the slower/faster buttons to change speed. Disappearance persists down to surprisingly low speeds. [If there are no buttons on the right, please update your Flash player.]

You can use the larger/smaller buttons to change size. Disappearance persists up to surprisingly large sizes.

You can use the “back-col” button to change the background colour. The yellow dots disappear into whatever colour the background has.

The ‘defaults’ button at the top restores the standard settings.

Comment
According to Michael Bach, "Steady fixation favours disappearance, blinks or gaze shifts induce reappearance. All in all reminiscent of the Troxler effect, but stronger and more resistant to residual eye movements."

Source
This page provided by Prof. Michael Bach PhD, Ophthalmology, University of Freiburg, Germany, from his collection of Optical Illusions & Visual Phenomena.