Alan, I just now processed two shots I made the week before with my Sony NEX 5N, opened in LR4, Process 2012, default settings are <NONE>, converted to B&W and deeply intensified the sky to make the images look dramatic, and they are just absolutely clean. No artifacts - just gorgeous, deep gray tonality. So I can't reproduce your problem within LR4 based on those samples. I think so much of this depends on the what kind of file it is, its characteristics when it starts life and what are the specific processing techniques. Much needs to be worked-through about those questions before suspecting the application. For example, when I did the B&W conversions and looked at them closely, I noticed a mild white fringe down one side of the structures against the sky. So I took them back into colour, magnified them greatly, and sure enough there was a bit of C-A that was hard to detect in the colour version, but was more obvious in the B&W.