I got this via artist and musician friend, Joseph Ekers, and it's not your average optical illusion. The claim is that square 'A' is exactly the same colour as square 'B'. I glance at the picture and think, 'no it's not, and there's an end to it.'
So I duplicate the image in Photoshop, and prepare to test the claim. (You might want to zoom in on these pics to get a better view.)
Using the cropping tool, I select a section of square 'A'...
...and begin to drag it downwards. At this point my section of 'A' still looks very different in colour to 'B'.
But as I drag it into the 'shadow' of the cylinder the tonal value appears to alter.
And as I reposition the section on the page, so that 'A' now covers 'B', I see that the two squares are indeed identical in both hue and tonal value. I see it, but I still can't quite believe it.
For an even simpler demonstration, here's the same pic with the surroundings of 'A' and 'B' removed.