ASUS G752VS, Windows 10 Pro (1511) 64bit, 32GB RAM, Nvidia GTX1070 8GB VRAM
Broken: 2.78c 64bit portable, 2.78.5 (Hash: 9e08019) 64bit portable
Short description of error
Adding an Image Texture node in the (by example) Diffuse BSDF shader from the button on the right of the Color field in the Material Properties panel adds instead the PxrProjectionLayer node.
Exact steps for others to reproduce the error
In Blender 2.78c 64bit (portable), as well as with the latest development build on Windows 10 Pro 64bit with the RMfB add-on activated (I have the latest version of both the add-on, which I sync with the Pixar's GitHub repository, as well as RMan 21.4) but not selected to be the active render engine, which is Cycles, if I add a new material like the default Diffuse BSDF node, and then I click on the Color input "dot" to open a list of nodes to choose from, selecting the Image Texture node Blender adds the PxrProjectionLayer node instead of the Image Texture node.
Here follow some screenshots:
Furthermore, I noticed that even if Cycles is the selected active render engine, all the nodes from PRMan are listed in the Add node menu, which in theory shouldn't.
Here is the link to the discussion on Blender Artists and Brian's comments, just in case.
Thank you for your attention and time.