I’m not sure if this is a bug or its suppose to look this way:
If you look at the tree trunk branch’s edge, there is a highlighted edge (see pointer in above screenshot) on the rendered frame .
This edge only gets highlighted where the spot lamp’s Halo cast goes over.
Is this normal?
It seems if I turn off Anti-Aliasing it goes away but then the scene does not look as good.
If you turn on “Full Sample” in the Anti-Aliasing Settings it should look better. This sometimes happens with alpha transparent objects.
Ok, thanks @2ndClemens I will try that. 
Update: yes it fixes the problem. But another issue comes in, apparently if I want to use motion blur (Sampled Motion Blur) I can not use that with Full Sample.
I think this may be the problem. 
https://developer.blender.org/T22537
So apparently I can’t use this in animations.
