When boundary of a mass world sensitive detector coincides with boundary of a parallel world sensitive detector, hit on this surface only calls one of these two sensitive detectors. A simple test can be: world of vacuum one box, made of vacuum, in this mass world one box in the parallel world, exactly the same size and location as the mass world box (e.g., they exactly overlap). One scorer attached to each of these boxes, such as the G4PSFlatSurfaceCurrent. When I shoot a particle at this geometry, I expect each scorer to get called at the same time. But what I see is that sometimes one scorer is called, while other steps the other scorer is called.
I looked into this bug again last month. I now believe the issue may simply be a problem in the verbose tracking output. It appears that both sensitive detectors are hit, but the output of verbose tracking doesn't show this properly. (my earlier tests complicated the study with some physics list problems. I apologize for the confusion). Here is what I see: Consider setup where I have identical volumes in n worlds. When the track gets to the volume, verbose tracking shows a step, i, starting on that volume. n-1 worlds get their scorers called with the hit i. Then verbose tracking shows one more step, i+1. And only then does the n'th world's scorer gets called, yet it is called with step i, not i+1. So the result is correct, all worlds get their scorer's called with hit i, but the diagnostics are very strange.
