The bug:
The vignette around the edges of the screen in dark areas has sharp angles rather than being a smooth radius around the screen. This results in noticeable lines where these angles meet.

How to reproduce

1. Dig down to y: 11 and dig a 1x2 tunnel 20 blocks long.
2. Place a torch at one end of the tunnel, and stand at the other end.
3. Walk towards the torch while focusing your attention on the edges of the screen.
4. Notice the sharp angles and artifacts.