The problem I encountered is that Geant 4.5.1 stops
(exits) whenever a trakc gets very close to a boundary
of a solid (I don't even know how it gets there).
For example:

----------------------------------------------------------
    *** Dump for solid - Shower ***
    ===================================================
 Solid type: G4Box
 Parameters:
    half length X: 42.5 mm
    half length Y: 42.5 mm
    half length Z: 180 mm
-----------------------------------------------------------
Position:

p.x() = 19.80463046276509 mm
p.y() = -42.50000000000002 mm
p.z() = -145.8308287146216 mm

Direction:

v.x() = 0
v.y() = 0
v.z() = 0

Proposed distance :

snxt = 9e+99 mm

G4Box::DistanceToOut() - Invalid enum

*** G4Exception: Aborting execution ***

Is there any trick to avoid such behaviour?

Thanks.

Wolfgang Korsch