we have some evidence of particles propagating outside the world volume. here is the original error report (please note that the Atlas volume is about 50m long) Already the second step (line 66) in "BPIPE::Section_01" through process "hIoni" leaves the Atlas envelope ( (x,y,z)=(-31.9 m,-71.1 m,-213 m) ). One of the secondaries produced (mu+, line 164 ff) steps 'forever', producing secondaries through "Atlas::Atlas MuIoni", in this outside region. The last recorded step before the crash is nr 1283645 at (x,y,z)=(-19.5 km,-44.6 km,-131 km).
Fixes have been introduced in release 6.0 which may help in fixing also such problem, although we do not have any real evidence of this through our test cases. Please, let us know if you still observe this.