I'm not sure if I should consider this one as bug or as design limitation:
When combining branched path tracing and sample clamping, the borders of overlapping volumes can become visible. This does not happen in plain path tracing or without sample clamping.
The root of this problem is that the number of volume samples is per volume segment and not per primary ray. In branched path tracing, camera rays that traverse multiple volumes have a higher probability of gathering scattered light than rays that go through only a single ray. Without clamping, different weights make this difference disappear when given enough AA samples. With clamping, the weighting loses effect and the different sample density is turned into a difference in intensity.