Ignore the horizontal lines, they seem to be part of a defect in the screenshot process.  These lines are not present in the game.

Otherwise, this is an example of the skybox problem.  This level is 'The Vast and Furious' created by ButterB quite a long time ago.  The cube-like vertex you see in the center upper third of the screen is where the rendering is not correct on the sky.  Nvidia drivers have a 'modified' version the the gl rendering command to deal with this. This image is from a system with an ATI All-in-Wonder 9600 Pro, with the latest 3.10 drivers.  The same happens with all drives from 3.7 up.