Is this after processing? It's quite typical when clarity especially is used more than a little. I very often see it with, for example black branches against a clear blue sky or some other almost silhouetted object (flag pole, chimney etc). I got round it by backing off global clarity (at high levels) and using the brush tool to paste it in where I needed it and making sure I didn't overflow in the sky. I've not done exhaustive tests but clarity in combination with vibrance might be a little worse
Examples of what you're seeing would help