Vertex snapping
What happens when vertex screen positions are rounded to whole pixels: polygons twitch in little jumps the moment the camera moves.
The original PlayStation (1994) did its geometry in fixed point and handed the GPU screen coordinates in whole pixels, with no subpixel precision. A vertex that should glide from x = 100.3 to 100.7 sits still, then leaps a full pixel. Thousands of vertices doing this independently make the entire scene shimmer, and seams between triangles briefly crack open.
It is often confused with affine texture mapping, the PS1's other signature flaw, but they are separate: one moves the corners, the other warps the texture between them. The Nintendo 64 and PC 3D cards had subpixel precision. Modern indie horror recreates the snapping on purpose with one line in a vertex shader.