If one place a small volume in a larger  one, both made of material Vacuum,
nothing determines the step length except the geometry. In this, case *without
 setting User Step Limit,

(detectLog->SetUserLimits(new G4UserLimits(2.0)))

the code falls into a infinite loop, after priting 100 wnaring message about the
step getting to small:

 Warning (G4MagIntegratorDriver): The stepsize for the  next
iteration=3.70591e-14 is too small - in Step number 1.
     Requested step size was 7411.83 .
     Previous  step size was 7411.83 .
     The minimum for the driver is 10
     The integrations has already gone 7.32747e-15

The current patch is to set a user step limit smaller than the volume, which
is not always optimal.

Here is an example source code:

// This code implementation is the intellectual property of
// the GEANT4 collaboration.
//
// By copying, distributing or modifying the Program (or any work
// based on the Program) you indicate your acceptance of this statement,
// and all its terms.
//
// $Id: Pr2DetectorConstruction.cc,v 1.1.10.1 1999/12/07 20:47:21
//gunter Exp $
// GEANT4 tag $Name: geant4-01-00 $
//

#include "Pr2DetectorConstruction.hh"

#include "G4Material.hh"
#include "G4Box.hh"
#include "G4Tubs.hh"
#include "G4LogicalVolume.hh"
#include "G4ThreeVector.hh"
#include "G4PVPlacement.hh"
#include "G4PVReplica.hh"
#include "globals.hh"
#include "G4UniformMagField.hh"
#include "G4FieldManager.hh"
#include "G4TransportationManager.hh"
#include "G4UserLimits.hh"
#include "G4EqMagElectricField.hh"
#include "G4ClassicalRK4.hh"
#include "G4SimpleRunge.hh"
#include "Pr2MagneticField.hh"

#include "dataCards.hh"

extern  Pr2GridSpace *AllGridsProton2[10];

Pr2DetectorConstruction::Pr2DetectorConstruction()
{;}

Pr2DetectorConstruction::~Pr2DetectorConstruction()
{;}

G4VPhysicalVolume* Pr2DetectorConstruction::Construct()
{

// Magnetic field
//---------------------------------------------------------------------------

 static G4bool fieldIsInitialized = false;
 if(!fieldIsInitialized)
   {
   int iFOpt =  (int) MyDataCards.fetchValueDouble("FieldOption");
   G4FieldManager* fieldMgr
     = G4TransportationManager::GetTransportationManager()
       ->GetFieldManager();
      Pr2MagneticField* fullEMField  =
                        new  Pr2MagneticField();
      G4EqMagElectricField *fEquationE =
        new G4EqMagElectricField(fullEMField);
      G4MagIntegratorStepper *pStepper = new G4ClassicalRK4 (fEquationE, 6);
//      G4MagIntegratorStepper *pStepper = new G4SimpleRunge (fEquationE, 6);
      fieldMgr->SetDetectorField(fullEMField);
      G4ChordFinder* pChordFinder =
             new G4ChordFinder(fullEMField, 10 * mm , pStepper);
      fieldMgr->SetChordFinder(pChordFinder);

// Testing..

      double point[3];
      double ff[6];
      point[0] = 5.23; point[1] = 8.56; point[2] = 300.;
      cout <<  " Testing at z = 300. \n";
      fullEMField->GetFieldValue( point, ff);
     for (int k=0; k<6; k++)
       cout << " Componnent " << k << " =  " << ff[k] << "\n";
    }
   // double newValue = 1*mm;
   //fieldMgr->GetChordFinder()->SetDeltaChord(newValue);
   fieldIsInitialized = true;
//------------------------------------------------------ materials

  double a;  // atomic mass
  double z;  // atomic number
  double density, pressure, temperature;
  G4String name;

  a = 12.01*g/mole;
  density = 2.265*g/cm3;
  G4Material* Carbon = new G4Material(name="Carbon", z=6., a, density);


  //Vacuum
    density     = universe_mean_density;    //from PhysicalConstants.h
    pressure    = 3.e-18*pascal;
    temperature = 2.73*kelvin;
    G4Material* Vacuum =
            new G4Material(name="Vacuum", z=1., a=1.01*g/mole, density,
                   kStateGas,temperature,pressure);

  //------------------------------------------------------ volumes
  double maxStep = 100.;
  double expHall_x = 500.*mm;
  double expHall_y = expHall_x;
  double expHall_z = 10000.0*mm;
  G4Box* experimentalHall_box
    = new G4Box("expHall_box",expHall_x,expHall_y, expHall_z);
  G4LogicalVolume* experimentalHall_log
    = new G4LogicalVolume(experimentalHall_box,Vacuum,"expHall_log",0,0,0);
  G4VPhysicalVolume* experimentalHall_phys
    = new G4PVPlacement(0,G4ThreeVector(),"expHall",
                        experimentalHall_log,0,false,0);
  experimentalHall_log->SetUserLimits(new G4UserLimits(maxStep));


  //---------------------- mini detector ------------------------

  double outerRad = 50.0*cm;
  double hight = 0.25*cm;
  G4Tubs* detectTube
    = new G4Tubs("Detect_tube",0.,
                 outerRad, hight, 0., 360.);
  G4LogicalVolume* detectLog
    = new G4LogicalVolume(detectTube,Vacuum,"DetectorLog",0,0,0);
//  G4LogicalVolume* detectLog
//    = new G4LogicalVolume(detectTube,Carbon,"DetectorLog",0,0,0);

// Slight offset, 2 cm before the target realy starts.

  G4VPhysicalVolume* det1_phys
    = new G4PVPlacement(0,
             G4ThreeVector(0., 0., - 20.*mm),
             "Detect_1", detectLog,experimentalHall_phys , false, 0);

//   detectLog->SetUserLimits(new G4UserLimits(2.0));

    return experimentalHall_phys;
}