Z-fighting
The flickering that appears when two surfaces sit so close in depth that the z-buffer cannot decide which one is in front.
When two planes coincide or nearly coincide, they end up with almost identical depth values. The z-buffer has finite precision, so which value wins comes down to rounding, and it can flip from pixel to pixel and frame to frame — the result is a strobing shimmer crawling across the surface.
It gets worse with distance. Perspective projection crams nearly all of its depth precision close to the camera, so the sliver that remains out at the far end of the draw distance simply isn't enough. The usual fixes are to push the surfaces apart, add a depth-bias term (polygon offset), tighten the near-to-far plane ratio, or flip the depth range (reversed-Z), which spreads precision more evenly across the scene.