Program: 55999
Project: 55999
Buggy source:
/** 
 * Returns a candidate vertex which could potentially be visited next because it is reachable from the currently considered vertex.
 * @param te the traversal entry for the current source vertex
 * @param forward <code>true</code> if the graph should be traversed in correct order, <code>false</code> to traverse it in reverse order
 * @return a candidate vertex which could potentially be visited next
 */
private ManagementVertex getCandidateVertex(final TraversalEntry te,final boolean forward){
  if (forward) {
    while (true) {
      if (this.confinedToStage && te.getCurrentChannel() == 0) {
        while (currentGateLeadsToOtherStage(te,this.forward)) {
          te.increaseCurrentGate();
        }
      }
      if (te.getCurrentGate() >= te.getManagementVertex().getNumberOfOutputGates()) {
        break;
      }
      if (te.getCurrentChannel() >= te.getManagementVertex().getOutputGate(te.getCurrentGate()).getNumberOfForwardEdges()) {
        te.increaseCurrentGate();
        te.resetCurrentChannel();
      }
 else {
        final ManagementEdge forwardEdge=te.getManagementVertex().getOutputGate(te.getCurrentGate()).getForwardEdge(te.getCurrentChannel());
        final ManagementVertex target=forwardEdge.getTarget().getVertex();
        te.increaseCurrentChannel();
        if (!alreadyVisited.contains(target)) {
          return target;
        }
      }
    }
  }
 else {
    while (true) {
      if (this.confinedToStage && te.getCurrentChannel() == 0) {
        while (currentGateLeadsToOtherStage(te,this.forward)) {
          te.increaseCurrentGate();
        }
      }
      if (te.getCurrentGate() >= te.getManagementVertex().getNumberOfInputGates()) {
        break;
      }
      if (te.getCurrentChannel() >= te.getManagementVertex().getInputGate(te.getCurrentGate()).getNumberOfBackwardEdges()) {
        te.increaseCurrentGate();
        te.resetCurrentChannel();
      }
 else {
        final ManagementEdge backwardEdge=te.getManagementVertex().getInputGate(te.getCurrentGate()).getBackwardEdge(te.getCurrentChannel());
        final ManagementVertex source=backwardEdge.getSource().getVertex();
        if (source == null) {
          LOG.error("String_Node_Str");
        }
        te.increaseCurrentChannel();
        if (!alreadyVisited.contains(source)) {
          return source;
        }
      }
    }
  }
  return null;
}

Fixed version (ground truth):
/** 
 * Returns a candidate vertex which could potentially be visited next because it is reachable from the currently considered vertex.
 * @param te the traversal entry for the current source vertex
 * @param forward <code>true</code> if the graph should be traversed in correct order, <code>false</code> to traverse it in reverse order
 * @return a candidate vertex which could potentially be visited next
 */
private ManagementVertex getCandidateVertex(final TraversalEntry te,final boolean forward){
  if (forward) {
    while (true) {
      if (this.confinedToStage && te.getCurrentChannel() == 0) {
        while (currentGateLeadsToOtherStage(te,this.forward)) {
          te.increaseCurrentGate();
        }
      }
      if (te.getCurrentGate() >= te.getManagementVertex().getNumberOfOutputGates()) {
        break;
      }
      if (te.getCurrentChannel() >= te.getManagementVertex().getOutputGate(te.getCurrentGate()).getNumberOfForwardEdges()) {
        te.increaseCurrentGate();
        te.resetCurrentChannel();
      }
 else {
        final ManagementEdge forwardEdge=te.getManagementVertex().getOutputGate(te.getCurrentGate()).getForwardEdge(te.getCurrentChannel());
        final ManagementVertex target=forwardEdge.getTarget().getVertex();
        te.increaseCurrentChannel();
        if (!alreadyVisited.contains(target)) {
          return target;
        }
      }
    }
  }
 else {
    while (true) {
      if (this.confinedToStage && te.getCurrentChannel() == 0) {
        while (currentGateLeadsToOtherStage(te,this.forward)) {
          te.increaseCurrentGate();
        }
      }
      if (te.getCurrentGate() >= te.getManagementVertex().getNumberOfInputGates()) {
        break;
      }
      if (te.getCurrentChannel() >= te.getManagementVertex().getInputGate(te.getCurrentGate()).getNumberOfBackwardEdges()) {
        te.increaseCurrentGate();
        te.resetCurrentChannel();
      }
 else {
        final ManagementEdge backwardEdge=te.getManagementVertex().getInputGate(te.getCurrentGate()).getBackwardEdge(te.getCurrentChannel());
        final ManagementVertex source=backwardEdge.getSource().getVertex();
        if (source == null) {
          LOG.error("String_Node_Str");
        }
        te.increaseCurrentChannel();
        if (!this.alreadyVisited.contains(source)) {
          return source;
        }
      }
    }
  }
  return null;
}