Error messages when tracking gammas: UnexpectedPositionShift

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Hello!

I am writing a fairly simple program to calculate the energy deposition distribution in a thin slice of silicon from different sources.

Right now I am shooting 180 GeV pions into the setup consiting of the target and some support materials. I am using the physics list from example N07 (modified to set the tracking cut down to 5 µm), and a few times I get this error message:

*********************************************************************************************************
* G4Track Information:   Particle = gamma,   Track ID = 27,   Parent ID = 23
*********************************************************************************************************

Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
    0     0.38  -0.0686      201    0.0751        0        0         0      LV_DUT initStep
    1     8.58    -1.67      202    0.0751        0     8.49      8.49    PV_world Transportation

*** G4Exception : UnexpectedPositionShift
      issued by : G4Navigator::ComputeStep()
Accuracy ERROR or slightly inaccurate position shift.
*** This is just a warning message.
     The Step's starting point has moved 202.493908 mm 
     since the last call to a Locate method.
     This has resulted in moving 8.491137061 mm  from the last point at which the safety      was calculated 
     which is more than the computed safety= 0 mm  at that point.
     This difference is 8.491137061 mm.
     The tolerated accuracy is 1e-06 mm.
  This problem can be due to either 
    - a process that has proposed a displacement larger than the current safety , or
    - inaccuracy in the computation of the safety
  We suggest that you 
   - find i) what particle is being tracked, and  ii) through what part of your geometry 
      for example by re-running this event with 
         /tracking/verbose 1 
    - check which processes you declare for this particle (and look at non-standard ones)
   - in case, create a detailed logfile of this event using:
         /tracking/verbose 6 
ERROR - G4Navigator::ComputeStep()
        Position has shifted considerably without notifying the navigator !
        Tolerated safety: 1e-06
        Computed shift  : 72.09940859

*** G4Exception : SignificantPositionShift
      issued by : G4Navigator::ComputeStep()
May lead to a crash or unreliable results.
*** This is just a warning message.
    2    1e+03     -196      383    0.0751        0 1.03e+03  1.03e+03  OutOfWorld Transportation

I only recive this error message when a photon is transported out of world, but has a stop in the world volume (air). What is going on? How can I fix it?