Afterimage - Negative of the flag of the Czech Republic. Stare at the image for ca 1 minute, then look at a white surface. You will see the Czech flag in real (positive) colours.
<a href="//commons.wikimedia.org/wiki/User:Lukas_Sova" title="User:Lukas Sova">Lukas Sova</a>
<span class="int-own-work" lang="en">Own work</span>