Cycles supports lights with "negative strength". However the light tree
didn't properly handle these lights. And under certain circumstances there
could be large increases in noise.

For example, in the scene I have attached below there are two lights
configured to be opposites of each other. The combined energy of these
lights in master is 1000 - 1000, which means the node these lights
occupy in the light tree has "zero energy" and the direct sampling
of these lights are skipped, which results in a large increase in noise.

Master	This Patch