Program: 22163
Project: 22163
Buggy source:
@Override public void safeUpdate(){
  super.superUpdate();
  EnumFacing dir=world.getBlockState(pos).getValue(BlockSteamHeater.FACING);
  ArrayList<TileEntitySteamHeater> secondaryHeaters=new ArrayList<>();
  BlockPos offsetPos=getOffsetPos(dir);
  TileEntity tile=world.getTileEntity(offsetPos);
  if (tile == null || !(tile instanceof TileEntityFurnace)) {
    return;
  }
  TileEntityFurnace furnace=(TileEntityFurnace)tile;
  int numHeaters=0;
  isPrimaryHeater=false;
  for (  EnumFacing dir2 : EnumFacing.VALUES) {
    int x=pos.getX() + dir.getFrontOffsetX() + dir2.getFrontOffsetX();
    int y=pos.getY() + dir.getFrontOffsetY() + dir2.getFrontOffsetY();
    int z=pos.getZ() + dir.getFrontOffsetZ() + dir2.getFrontOffsetZ();
    BlockPos pos2=new BlockPos(x,y,z);
    TileEntity tile2=world.getTileEntity(pos2);
    IBlockState state2=world.getBlockState(pos2);
    if (tile2 != null) {
      if (tile2 instanceof TileEntitySteamHeater) {
        TileEntitySteamHeater heater2=(TileEntitySteamHeater)tile2;
        if (heater2.getSteamShare() >= CONSUMPTION && state2.getValue(BlockSteamHeater.FACING).getOpposite() == dir2) {
          isPrimaryHeater=x == pos.getX() && y == pos.getY() && z == pos.getZ();
          secondaryHeaters.add(heater2);
          numHeaters++;
          if (secondaryHeaters.size() > 4) {
            secondaryHeaters.remove(0);
          }
          numHeaters=Math.min(4,numHeaters);
        }
      }
    }
  }
  if (isPrimaryHeater && numHeaters > 0) {
    if (!(furnace instanceof TileEntitySteamFurnace) && furnace.getClass() == TileEntityFurnace.class) {
      replace(furnace);
    }
    if (!(furnace instanceof TileEntitySteamFurnace)) {
      return;
    }
    int furnaceBurnTime=furnace.getField(FURNACE_BURN_TIME_ID);
    int furnaceCookTime=furnace.getField(COOK_TIME_ID);
    if ((furnaceBurnTime == 1 || furnaceBurnTime == 0) && getSteamShare() >= CONSUMPTION && ((TileEntitySteamFurnace)furnace).canSmelt()) {
      if (furnaceBurnTime == 0) {
        BlockFurnace.setState(true,world,offsetPos);
      }
      for (      TileEntitySteamHeater heater : secondaryHeaters) {
        heater.decrSteam(CONSUMPTION);
      }
      furnace.setField(0,furnaceBurnTime + 3);
      if (numHeaters > 1 && furnaceCookTime > 0) {
        int newCookTime=Math.min(furnaceCookTime + 2 * numHeaters - 1,199);
        furnace.setField(COOK_TIME_ID,newCookTime);
      }
      world.notifyBlockUpdate(offsetPos,world.getBlockState(offsetPos),world.getBlockState(offsetPos),0);
    }
  }
}

Fixed version (ground truth):
@Override public void safeUpdate(){
  superUpdate();
  EnumFacing dir=world.getBlockState(pos).getValue(BlockSteamHeater.FACING);
  List<TileEntitySteamHeater> secondaryHeaters=new ArrayList<>();
  BlockPos offsetPos=getOffsetPos(dir);
  TileEntity tile=world.getTileEntity(offsetPos);
  if (!(tile instanceof TileEntityFurnace)) {
    return;
  }
  TileEntityFurnace furnace=(TileEntityFurnace)tile;
  int numHeaters=0;
  isPrimaryHeater=false;
  for (  EnumFacing dir2 : EnumFacing.VALUES) {
    int x=pos.getX() + dir.getFrontOffsetX() + dir2.getFrontOffsetX();
    int y=pos.getY() + dir.getFrontOffsetY() + dir2.getFrontOffsetY();
    int z=pos.getZ() + dir.getFrontOffsetZ() + dir2.getFrontOffsetZ();
    BlockPos pos2=new BlockPos(x,y,z);
    TileEntity tile2=world.getTileEntity(pos2);
    IBlockState state2=world.getBlockState(pos2);
    if (tile2 != null) {
      if (tile2 instanceof TileEntitySteamHeater) {
        TileEntitySteamHeater heater2=(TileEntitySteamHeater)tile2;
        if (heater2.getSteamShare() >= CONSUMPTION && state2.getValue(BlockSteamHeater.FACING).getOpposite() == dir2) {
          isPrimaryHeater=x == pos.getX() && y == pos.getY() && z == pos.getZ();
          secondaryHeaters.add(heater2);
          numHeaters++;
          if (secondaryHeaters.size() > 4) {
            secondaryHeaters.remove(0);
          }
          numHeaters=Math.min(4,numHeaters);
        }
      }
    }
  }
  if (isPrimaryHeater && numHeaters > 0) {
    if (!(furnace instanceof TileEntitySteamFurnace) && furnace.getClass() == TileEntityFurnace.class) {
      replace(furnace);
    }
    if (!(furnace instanceof TileEntitySteamFurnace)) {
      return;
    }
    int furnaceBurnTime=furnace.getField(FURNACE_BURN_TIME_ID);
    int furnaceCookTime=furnace.getField(COOK_TIME_ID);
    if ((furnaceBurnTime == 1 || furnaceBurnTime == 0) && getSteamShare() >= CONSUMPTION && ((TileEntitySteamFurnace)furnace).canSmelt()) {
      if (furnaceBurnTime == 0) {
        BlockFurnace.setState(true,world,offsetPos);
      }
      for (      TileEntitySteamHeater heater : secondaryHeaters) {
        heater.decrSteam(CONSUMPTION);
      }
      furnace.setField(0,furnaceBurnTime + 3);
      if (numHeaters > 1 && furnaceCookTime > 0) {
        int newCookTime=Math.min(furnaceCookTime + 2 * numHeaters - 1,199);
        furnace.setField(COOK_TIME_ID,newCookTime);
      }
      world.notifyBlockUpdate(offsetPos,world.getBlockState(offsetPos),world.getBlockState(offsetPos),0);
    }
  }
}