Program: 190860
Project: 190860
Buggy source:
@Override public Set<? extends Cluster<T>> getClusters(double zoom){
  final int discreteZoom=(int)zoom;
  final double zoomSpecificSpan=MAX_DISTANCE_AT_ZOOM / Math.pow(2,discreteZoom) / 256;
  final Set<QuadItem<T>> visitedCandidates=new HashSet<QuadItem<T>>();
  final Set<Cluster<T>> results=new HashSet<Cluster<T>>();
  final Map<QuadItem<T>,Double> distanceToCluster=new HashMap<QuadItem<T>,Double>();
  final Map<QuadItem<T>,StaticCluster<T>> itemToCluster=new HashMap<QuadItem<T>,StaticCluster<T>>();
  for (  QuadItem<T> candidate : mItems) {
    if (visitedCandidates.contains(candidate)) {
      continue;
    }
    Bounds searchBounds=createBoundsFromSpan(candidate.getPoint(),zoomSpecificSpan);
    Collection<QuadItem<T>> clusterItems;
synchronized (mQuadTree) {
      clusterItems=mQuadTree.search(searchBounds);
    }
    if (clusterItems.size() == 1) {
      results.add(candidate);
      visitedCandidates.add(candidate);
      distanceToCluster.put(candidate,0d);
      continue;
    }
    StaticCluster<T> cluster=new StaticCluster<T>(candidate.mClusterItem.getPosition());
    results.add(cluster);
    for (    QuadItem<T> clusterItem : clusterItems) {
      Double existingDistance=distanceToCluster.get(clusterItem);
      double distance=distanceSquared(clusterItem.getPoint(),candidate.getPoint());
      if (existingDistance != null) {
        if (existingDistance < distance) {
          continue;
        }
        itemToCluster.get(clusterItem).remove(clusterItem.mClusterItem);
      }
      distanceToCluster.put(clusterItem,distance);
      cluster.add(clusterItem.mClusterItem);
      itemToCluster.put(clusterItem,cluster);
    }
    visitedCandidates.addAll(clusterItems);
  }
  return results;
}

Fixed version (ground truth):
@Override public Set<? extends Cluster<T>> getClusters(double zoom){
  final int discreteZoom=(int)zoom;
  final double zoomSpecificSpan=MAX_DISTANCE_AT_ZOOM / Math.pow(2,discreteZoom) / 256;
  final Set<QuadItem<T>> visitedCandidates=new HashSet<QuadItem<T>>();
  final Set<Cluster<T>> results=new HashSet<Cluster<T>>();
  final Map<QuadItem<T>,Double> distanceToCluster=new HashMap<QuadItem<T>,Double>();
  final Map<QuadItem<T>,StaticCluster<T>> itemToCluster=new HashMap<QuadItem<T>,StaticCluster<T>>();
synchronized (mQuadTree) {
    for (    QuadItem<T> candidate : mItems) {
      if (visitedCandidates.contains(candidate)) {
        continue;
      }
      Bounds searchBounds=createBoundsFromSpan(candidate.getPoint(),zoomSpecificSpan);
      Collection<QuadItem<T>> clusterItems;
      clusterItems=mQuadTree.search(searchBounds);
      if (clusterItems.size() == 1) {
        results.add(candidate);
        visitedCandidates.add(candidate);
        distanceToCluster.put(candidate,0d);
        continue;
      }
      StaticCluster<T> cluster=new StaticCluster<T>(candidate.mClusterItem.getPosition());
      results.add(cluster);
      for (      QuadItem<T> clusterItem : clusterItems) {
        Double existingDistance=distanceToCluster.get(clusterItem);
        double distance=distanceSquared(clusterItem.getPoint(),candidate.getPoint());
        if (existingDistance != null) {
          if (existingDistance < distance) {
            continue;
          }
          itemToCluster.get(clusterItem).remove(clusterItem.mClusterItem);
        }
        distanceToCluster.put(clusterItem,distance);
        cluster.add(clusterItem.mClusterItem);
        itemToCluster.put(clusterItem,cluster);
      }
      visitedCandidates.addAll(clusterItems);
    }
  }
  return results;
}