Scott Elliott sent us this computer screen illusion. He says that he has not found a definitive discussion of it anywhere.
The picture shows a red-white checkerboard with some squares replaced with magenta-yellow in a group shaped like the word Hi.
At close range it's easy to distinguish the red-white groups from the yellow-magenta ones, but if you back away from your screen they will start to blend together. In itself that's not so surprising because that's how printed halftones work in color publications, but an unforeseen transformation occurs long before the colors blend: the yellow-magenta squares disguise themselves as red-white squares, but otherwise the checkerboard remains clearly defined. At some distance beyond 2 meters (depending on your display size) it will appear unambiguously as a red-white grid.
Alternatively, ask someone to describe what they see when they view it at a distance: they'll report a clearly-defined checkerboard of red and white squares. As they approach it they will gradually detect a discolored tinge, then distinguish the presence of two more tints - yellow and magenta.
Scott tells us that he chanced across this phenomenon around 1991 while trying the Paint program included with Microsoft Windows 3.0.