z-fighting

Z-fighting, also called stitching, is a phenomenon in 3D rendering that occurs when two or more primitives have similar values in the z-buffer.  This problem is usually caused by limited sub-pixel precision and floating point and fixed point round-off errors. https://en.wikipedia.org/wiki/Z-fighting

 

There is very high precision at the near plane, but very little precision at the far plane. If the range [-n, -f] is getting larger, it causes a depth precision problem (z-fighting); a small change of z_e around the far plane does not affect on z_n value. The distance between n and f should be short as possible to minimize the depth buffer precision problem.

http://www.songho.ca/opengl/gl_projectionmatrix.html