Program: 34867
Project: 34867
Buggy source:
@Override public void updateEntity(){
  super.updateEntity();
  if (worldObj.isRemote)   return;
  ItemStack stack=inventory.getStackInSlot(0);
  if (stack != null) {
    Item stackItem=stack.getItem();
    if (stackItem instanceof IFluidContainerItem) {
      IFluidContainerItem iFluidContainerItem=(IFluidContainerItem)stackItem;
      if (fill) {
        if (!tank.isEmpty()) {
          int amount=128;
          if (tank.getFluidAmount() < amount)           amount=tank.getFluidAmount();
          if (energy >= amount) {
            drain(ForgeDirection.UNKNOWN,iFluidContainerItem.fill(stack,new FluidStack(tank.getFluid().fluidID,amount),true),true);
            energy-=amount;
          }
        }
      }
 else {
        FluidStack contained=iFluidContainerItem.getFluid(stack);
        if (!fill && !tank.isFull() && contained != null && contained.amount > 0) {
          int amount=64;
          if (tank.getFreeSpace() < amount)           amount=tank.getFreeSpace();
          if (amount > contained.amount)           amount=contained.amount;
          iFluidContainerItem.drain(stack,fill(ForgeDirection.UNKNOWN,new FluidStack(contained.fluidID,amount),true),true);
        }
      }
    }
 else     if (FluidContainerRegistry.isContainer(stack)) {
      if (fill) {
        if (!tank.isEmpty()) {
          int amount=FluidContainerRegistry.getContainerCapacity(tank.getFluid(),stack);
          if (amount > 0 && energy >= amount && tank.getFluidAmount() >= amount) {
            ItemStack filledContainer=FluidContainerRegistry.fillFluidContainer(new FluidStack(tank.getFluid().fluidID,amount),stack);
            if (filledContainer != null && filledContainer.getItem() != null && filledContainer.stackSize > 0) {
              energy-=amount;
              drain(ForgeDirection.UNKNOWN,amount,true);
              inventory.setInventorySlotContents(0,filledContainer.copy());
            }
          }
        }
      }
 else {
        FluidStack contained=FluidContainerRegistry.getFluidForFilledItem(stack);
        if (contained != null && contained.amount > 0 && tank.getFreeSpace() >= contained.amount) {
          if (fill(ForgeDirection.UNKNOWN,contained,false) == contained.amount) {
            fill(ForgeDirection.UNKNOWN,contained,true);
            ItemStack drainedContainer=FluidContainerRegistry.drainFluidContainer(stack);
            if (drainedContainer != null && drainedContainer.getItem() != null && drainedContainer.stackSize > 0)             inventory.setInventorySlotContents(0,drainedContainer.copy());
          }
        }
      }
    }
    if (getProgress() >= 16) {
      stack=getStackInSlot(0);
      ItemStack outputStack=getStackInSlot(1);
      if (outputStack == null) {
        ItemStack copyStack=stack.copy();
        copyStack.stackSize=1;
        inventory.setInventorySlotContents(1,copyStack);
        inventory.decrStackSize(0,1);
      }
 else       if (Utils.areItemStacksMergeable(stack,outputStack) && outputStack.stackSize + stack.stackSize <= outputStack.getMaxStackSize()) {
        outputStack.stackSize+=stack.stackSize;
        inventory.decrStackSize(0,1);
      }
    }
  }
}

Fixed version (ground truth):
@Override public void updateEntity(){
  super.updateEntity();
  if (worldObj.isRemote)   return;
  ItemStack stack=inventory.getStackInSlot(0);
  if (stack != null) {
    Item stackItem=stack.getItem();
    if (stackItem instanceof IFluidContainerItem) {
      IFluidContainerItem iFluidContainerItem=(IFluidContainerItem)stackItem;
      if (fill) {
        if (!tank.isEmpty()) {
          int amount=128;
          if (tank.getFluidAmount() < amount)           amount=tank.getFluidAmount();
          if (energy >= amount) {
            drain(ForgeDirection.UNKNOWN,iFluidContainerItem.fill(stack,new FluidStack(tank.getFluid().fluidID,amount),true),true);
            energy-=amount;
          }
        }
      }
 else {
        FluidStack contained=iFluidContainerItem.getFluid(stack);
        if (!fill && !tank.isFull() && contained != null && contained.amount > 0) {
          int amount=64;
          if (tank.getFreeSpace() < amount)           amount=tank.getFreeSpace();
          if (amount > contained.amount)           amount=contained.amount;
          iFluidContainerItem.drain(stack,fill(ForgeDirection.UNKNOWN,new FluidStack(contained.fluidID,amount),true),true);
        }
      }
    }
 else     if (FluidContainerRegistry.isContainer(stack)) {
      if (fill) {
        if (!tank.isEmpty()) {
          int amount=FluidContainerRegistry.getContainerCapacity(tank.getFluid(),stack);
          if (amount > 0 && energy >= amount && tank.getFluidAmount() >= amount) {
            ItemStack filledContainer=FluidContainerRegistry.fillFluidContainer(new FluidStack(tank.getFluid().fluidID,amount),stack);
            if (filledContainer != null && filledContainer.getItem() != null && filledContainer.stackSize > 0) {
              energy-=amount;
              drain(ForgeDirection.UNKNOWN,amount,true);
              inventory.setInventorySlotContents(0,filledContainer.copy());
            }
          }
        }
      }
 else {
        FluidStack contained=FluidContainerRegistry.getFluidForFilledItem(stack);
        if (contained != null && contained.amount > 0 && tank.getFreeSpace() >= contained.amount) {
          if (fill(ForgeDirection.UNKNOWN,contained,false) == contained.amount) {
            fill(ForgeDirection.UNKNOWN,contained,true);
            ItemStack drainedContainer=FluidContainerRegistry.drainFluidContainer(stack);
            if (drainedContainer != null && drainedContainer.getItem() != null && drainedContainer.stackSize > 0)             inventory.setInventorySlotContents(0,drainedContainer.copy());
          }
        }
      }
    }
    if (getProgress() >= 16) {
      stack=getStackInSlot(0);
      if (stack != null) {
        ItemStack outputStack=getStackInSlot(1);
        if (outputStack == null || outputStack.getItem() == null || outputStack.stackSize <= 0) {
          ItemStack copyStack=stack.copy();
          copyStack.stackSize=1;
          inventory.setInventorySlotContents(1,copyStack);
          inventory.decrStackSize(0,1);
        }
      }
    }
  }
}