  We study the differences in the gamma ray spectra simulated by four Monte
Carlo event generator packages developed in particle physics. Two different
versions of PYTHIA and two of HERWIG are analyzed, namely PYTHIA 6.418 and
HERWIG 6.5.10 in Fortran and PYTHIA 8.165 and HERWIG 2.6.1 in C++. For all the
studied channels, the intrinsic differences between them are shown to be
significative and may play an important role in misunderstanding dark matter
signals.
