Why does my render quality decrease when I open my file in an older version, then bring it back to the current version? I'm working on a project in Blender 2.78. Normally I work from my fancy workstation. Last night, I pulled up my project on my laptop in Blender 2.76b (that's the newest version that works on my laptop). I did some more modeling and saved the file. Today, I opened the file up in 2.78 again and the render quality has dropped dramatically. I'm using an environment map for lighting. Even when I pump the samples up, the quality is horrible. When I create an area light with an emission shader, the quality is as expected, but the environment light has suffered. 

Here's a screenshot that compares the render quality using an environment texture for lighting. Notice the quality difference and the number of samples in each image:



Why has my render quality dropped? 

Can I fix my file or should I create a new .blend and append the objects from my original file? (that seems to fix the problem, but it's not an ideal solution since I will probably be doing more work on my laptop).
Check the "multiple importance sampling" option in the world panel. Sometime recently, I believe post-2.76, it was changed to be enabled by default. Disabling it would produce the result you see here, and might happen if you change Blender versions without explicitly setting that flag.

See this answer for what that setting does: https://blender.stackexchange.com/a/8156/8081