Unwanted black frames around fog material So I was following this Tutorial; https://www.youtube.com/watch?v=72LPW4S8bns&amp;t=1263s
then I was creating the fogs but suddenly the fog boxes that are further from camera started getting black corners. 
Im posting the nodes, please take a look
The problem and the solution to it is mentioned in the video you are linking to (appr. 20:10).

You have to increase the max bounces for Transparency under Render Properties / Light Paths / Max Bounces until the "black boxes" doesn't show up anymore.