I was using precompiled 5.2.p01 release libraries for RedHat7.3-gcc3.2
downloaded from the geant4 website on a RedHat7.3 system but I suspect this
applies to many other releases as well.

While investigating bogus warning messages that looked like

  G4ParticleGun::pi+
  KineticEnergy and Momentum could be inconsistent
 (Momentum:0.291109 GeV/c  Mass:0.13957 GeV/c/c)
  KineticEnergy is overwritten!! 0.182452->0.183268GeV

from void G4ParticleGun::SetParticleMomentum(G4ParticleMomentum)

(which you can reproduce by adding a call to this function anywhere in user
code), I tracked down the problem to the following piece of code

    G4double mass = particle_definition->GetPDGMass();
    G4double p = aMomentum.mag();
    particle_momentum_direction = aMomentum.unit();
    if ((particle_energy>0.0)
      &&(abs(particle_energy+mass-sqrt(p*p+mass*mass))>keV))
    {
      ...
      G4cout << "  KineticEnergy and Momentum could be inconsistent"
      ...
    }

In general, when you write abs(x) where x is a double, what you get
depends on various factors: the expression it's used in, the operating
system, compiler and version, whether you '#include <cmath>' or are
'using namespace std', phase of the moon, etc. In the code snippet above, the
abs() silently truncates its double argument to an integer. I think it's always
better to use fabs in these cases to avoid confusion although one could probably
find some magic combination of compiler switches, etc. to get the right
behavior. Here, it results in a harmless warning but to check where else this
might be a real problem, I ran

  cd $G4INSTALL/source
  find . -regex '.*\.i?cc' -print | xargs grep -l '[^f]abs('

which returns a total of 171 hits on the 6.0 source tree (I am not including the
output since you can run the command to find out). I looked quickly through
about half the files in the list and 3/4 of them seemed to have the same misuse
of abs() instead of fabs(). I don't have time to enter a hundred bug reports but
somebody should go through the list and have them fixed, the files are
distributed in many components (geometry, physics, etc.).

It's disappointing to see such basic mistakes creep into the project
and go through whatever quality control/testing mechanisms there are
(this would surely have generated a compiler warning in most places and various
distributions in physics code would come out wrong in simple tests).