In tag Geant4-01-01,
processes/electromagnetic/standard/src/G4MultipleScattering.cc,
line 459, the formula for "prob" has a logarithmic singularity at tau=1:

 else
    {
      tau = truestep/range ;
      if(tau < taulim)
        prob = exp(-(alpha1-1.)*tau)*(1.+scatteringparameter*tau) ;
      else
here===> prob = exp((alpha1-1.)*log(1.-tau))*(1.+scatteringparameter*tau) ;
    }


I think this should be changed to
      if (tau < taulim)
        prob = exp(-(alpha1-1.)*tau)*(1.+scatteringparameter*tau) ;
      else if (tau < 1.)
        prob = exp((alpha1-1.)*log(1.-tau))*(1.+scatteringparameter*tau) ;
      else
        prob = 0;