Program: 134256
Project: 134256
Buggy source:
@Override public void transformBlock(World world,Random random,Phase phase,BlockCoord coord,Block sourceBlock,int sourceMetadata){
  BiomeGenBase biome=world.getBiomeGenForCoords(coord.x,coord.z);
  Block topBlock=biome.topBlock;
  Block fillerBlock=biome.fillerBlock;
  coord=coord.subtract(0,4,0);
  int currentY=coord.y;
  List<int[]> currentList=new ArrayList<>();
  List<int[]> nextList=new ArrayList<>();
  nextList.add(new int[]{coord.x,coord.z});
  int worldHeight=world.getHeight();
  while (nextList.size() > 0 && currentY < worldHeight) {
    List<int[]> cachedList=currentList;
    currentList=nextList;
    nextList=cachedList;
    while (currentList.size() > 0) {
      int[] currentPos=currentList.remove(0);
      int currentX=currentPos[0];
      int currentZ=currentPos[1];
      Block curBlock=world.getBlock(currentX,currentY,currentZ);
      boolean isFoliage=curBlock.isFoliage(world,currentX,currentY,currentZ) || curBlock.getMaterial() == Material.leaves || curBlock.getMaterial() == Material.plants || curBlock.getMaterial() == Material.wood;
      boolean isCommon=curBlock == Blocks.stone || curBlock == Blocks.dirt || curBlock == Blocks.sand || curBlock == Blocks.stained_hardened_clay || curBlock == Blocks.gravel;
      boolean replaceable=currentY == coord.y || curBlock == topBlock || curBlock == fillerBlock || curBlock.isReplaceable(world,currentX,currentY,currentZ) || isCommon || isFoliage;
      if (replaceable) {
        world.setBlockToAir(currentX,currentY,currentZ);
      }
      if (replaceable || curBlock.getMaterial() == Material.air) {
        double distToOrigSQ=IvVecMathHelper.distanceSQ(new double[]{coord.x,coord.y,coord.z},new double[]{currentX,currentY,currentZ});
        double add=(random.nextDouble() - random.nextDouble()) * NATURAL_DISTANCE_RANDOMIZATION;
        distToOrigSQ+=add < 0 ? -(add * add) : (add * add);
        if (distToOrigSQ < NATURAL_EXPANSION_DISTANCE * NATURAL_EXPANSION_DISTANCE) {
          addIfNew(nextList,currentX,currentZ);
          addIfNew(nextList,currentX - 1,currentZ);
          addIfNew(nextList,currentX + 1,currentZ);
          addIfNew(nextList,currentX,currentZ - 1);
          addIfNew(nextList,currentX,currentZ + 1);
        }
      }
    }
    currentY++;
  }
}

Fixed version (ground truth):
@Override public void transformBlock(World world,Random random,Phase phase,BlockCoord coord,Block sourceBlock,int sourceMetadata){
  BiomeGenBase biome=world.getBiomeGenForCoords(coord.x,coord.z);
  Block topBlock=biome.topBlock != null ? biome.topBlock : Blocks.air;
  Block fillerBlock=biome.fillerBlock != null ? biome.fillerBlock : Blocks.air;
  coord=coord.subtract(0,4,0);
  int currentY=coord.y;
  List<int[]> currentList=new ArrayList<>();
  List<int[]> nextList=new ArrayList<>();
  nextList.add(new int[]{coord.x,coord.z});
  int worldHeight=world.getHeight();
  while (nextList.size() > 0 && currentY < worldHeight) {
    List<int[]> cachedList=currentList;
    currentList=nextList;
    nextList=cachedList;
    while (currentList.size() > 0) {
      int[] currentPos=currentList.remove(0);
      int currentX=currentPos[0];
      int currentZ=currentPos[1];
      Block curBlock=world.getBlock(currentX,currentY,currentZ);
      boolean isFoliage=curBlock.isFoliage(world,currentX,currentY,currentZ) || curBlock.getMaterial() == Material.leaves || curBlock.getMaterial() == Material.plants || curBlock.getMaterial() == Material.wood;
      boolean isCommon=curBlock == Blocks.stone || curBlock == Blocks.dirt || curBlock == Blocks.sand || curBlock == Blocks.stained_hardened_clay || curBlock == Blocks.gravel;
      boolean replaceable=currentY == coord.y || curBlock == topBlock || curBlock == fillerBlock || curBlock.isReplaceable(world,currentX,currentY,currentZ) || isCommon || isFoliage;
      if (replaceable) {
        world.setBlockToAir(currentX,currentY,currentZ);
      }
      if (replaceable || curBlock.getMaterial() == Material.air) {
        double distToOrigSQ=IvVecMathHelper.distanceSQ(new double[]{coord.x,coord.y,coord.z},new double[]{currentX,currentY,currentZ});
        double add=(random.nextDouble() - random.nextDouble()) * NATURAL_DISTANCE_RANDOMIZATION;
        distToOrigSQ+=add < 0 ? -(add * add) : (add * add);
        if (distToOrigSQ < NATURAL_EXPANSION_DISTANCE * NATURAL_EXPANSION_DISTANCE) {
          addIfNew(nextList,currentX,currentZ);
          addIfNew(nextList,currentX - 1,currentZ);
          addIfNew(nextList,currentX + 1,currentZ);
          addIfNew(nextList,currentX,currentZ - 1);
          addIfNew(nextList,currentX,currentZ + 1);
        }
      }
    }
    currentY++;
  }
}