Win 7 x64
Broken in master as of Sept 9 2015
Short description of error
When adding a material in your scene that uses the Is Shadow Ray light path as a mix factor, viewport rendering doesn't update correctly. Stopping and restarting rendering causes it to update correctly, and it updates correctly in all cases after the original error.
Exact steps for others to reproduce the error
Doesn't seem to work with a saved/loaded file, sorry.
My steps to recreate are: create a new file, create a sphere on top of a plane, add sun lamp, start preview rendering, add material to sphere with diffuse and transparent mixed via Is Shadow Ray.
Image one shows result after above steps, image two shows the result after hitting shift + z twice