We might all have suggestions as to what this might be, but for me its purple fringing or for landscape photographers, the purple branch effect. Its been attributed to lenses, processing, or apertures. It might, or might not, be a form of chromatic aberration. Some lenses and cameras show less of it, but I haven't yet found one lens or one camera that is immune from it. The example at the top is from a Leica M9 and Zeiss 18mm. The Zeiss is extremely good for "normal" chromatic aberration, but still renders the branches purple, on this out of camera jpg.
A photoshop conversion is slightly better, ticking the defringe all edges box in Adobe Camera Raw.
Still not anywhere near perfect though. By selecting the offending colour in Photoshop and using replace colour I can get it looking better.
At least the branches look close to their real colour now. However the saturation in the sky has been toned down, and there is a slight halo effect around the branches.
This is not a new phenomenum and is not restricted to digital photography. Now being able to watch movies in high definition has highlighted just how long standing this problem is. If ever I'm watching a film with bare branches against a blue sky I usually see this same phenomenon. Considering that film makers often use incredibly expensive lenses, its not down to how much you pay.
I've never found a satisfactory fix for it, nor have I ever found anyone else who has an answer for it. It also seems very unpredictable. If anyone out there has an answer to this, let me know and I'll put it in a post.