Sometimes I encouter this problem. Anyone knows why this happens and how to fix it?
Here you can download the blend file: https://www.dropbox.com/sh/dkoikr6qwosq25f/AADJYnH1-uv2haNshs-nq7nWa?dl=0
Flat Shading. Top one is the problem
Normals are all facing the same direction
Artifacts also happens in Smooth shading
With Subdivision modifier, the problem is gone. But why?