I've tracked down an apparently infinite looping job in our current production.  The tail-end of a run with debug output is left below, where you can see that after step 12 the anti-proton being tracked gets stuck.

It's in the forward calorimeter, in a volume called LArMgr::ModeratorTube, which is made of BoratedPolyethelyne, which has a density of 0.99 and a composition of:

Boron	0.05
Carbon	0.813
Hydrogen	0.137

I've gotten the thing running locally for a few hours now on the same step, so as usual, I suspect a 'while' without some protection.

From the stack, I gather that the thing is getting stuck in G4QEnvironment::FSInteraction() , since I see no loop in G4QEnvironment::Fragment() or G4QCaptureAtRest::AtRestDoIt(...) that makes me too terribly suspicious.  But FSInteraction is about 4000 lines long, so I hope you'll forgive me if I'm reluctant to try my usual reading and digging to spot the problem.

This is with G4 9.4p1, and it is quite rare (order 1 per 1.5M events).

Best,
Zach

* G4Track Information:   Particle = anti_proton,   Track ID = 3565283,   Parent ID = 3565268
*********************************************************************************************************

Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
  0      -29      151 -3.71e+03       177        0        0         0 LArMgr::ModeratorTube initStep
  1    -32.4      151 -3.74e+03       161     15.4     29.5      29.5 LArMgr::ModeratorTube hadElastic
  2    -38.5      147 -3.77e+03       141     19.8     36.2      65.7 LArMgr::ModeratorTube hIoni
  3    -43.1      144 -3.8e+03       123     18.2     29.1      94.8 LArMgr::ModeratorTube hIoni
  4    -47.1      141 -3.82e+03       107     15.9     23.2       118 LArMgr::ModeratorTube hIoni
  5    -50.4      139 -3.84e+03      93.5     13.5     18.5       136 LArMgr::ModeratorTube hIoni
  6    -52.8      138 -3.86e+03      81.2     12.3     14.9       151 LArMgr::ModeratorTube hIoni
  7    -54.5      136 -3.87e+03      70.3       11     11.9       163 LArMgr::ModeratorTube hIoni
  8    -56.1      135 -3.88e+03      60.1     10.2     9.53       173 LArMgr::ModeratorTube hIoni
  9    -57.4      135 -3.89e+03      51.1     8.97     7.57       180 LArMgr::ModeratorTube hIoni
 10    -58.4      134 -3.89e+03      42.7     8.38     6.04       186 LArMgr::ModeratorTube hIoni
 11    -59.4      134 -3.9e+03      35.3     7.47     4.79       191 LArMgr::ModeratorTube hIoni