macOS 10.12.3 NVIDIA GeForce GT 755M 1024 MB
Broken: 2.78b 2017-02-03 16:54 hash 9cb21a1
Short description of error
Blender does not seem to find the correct target image for a bake if there are multiple materials with image textures.
Exact steps for others to reproduce the error
A mesh with several materials assigned, one per face.
Each material has an image texture.
An additional blank bake target image
Each material also has a second, unconnected image texture for that target bake, and that image texture is selected in each material.
Going to the render tab and clicking "Bake" (a combined bake) produces a circular dependency error from one of the image textures to the object, and the image texture in question is overwritten by the bake, even though each material has a second image texture for the bake, and that second target image texture is selected in each material.