You may notice the following problems in environment fog in a rendered image.
Jagged surface edges in environment fog
Outlines around 2D motion blurred surfaces
Grainy or flickering environment fog
Break Connection on Environment Fog does not delete fog
If you have turned on Environment Fog in the Render Settings window, and try to turn it off later using right-mouse button Break Connection, the fog remains.
Find the Environment Fog material in Hypershade and delete it.