So i was working on another mod and i typed the same code for the ISidedInventory methods and tested.  the slot dupes the itemstack you put in there and when i try to take it out it disapears.  I even tried copying the code from one of my mods and it still does the same thing.

TileEntity:

 

 

package info.coremodding.realsmelting.tileentities;

import info.coremodding.realsmelting.blocks.LavaFurnace;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.init.Blocks;
import net.minecraft.init.Items;
import net.minecraft.inventory.ISidedInventory;
import net.minecraft.item.ItemStack;
import net.minecraft.item.crafting.FurnaceRecipes;
import net.minecraft.tileentity.TileEntity;

public class TileEntityLavaFurnace extends TileEntity implements ISidedInventory{

private static final int[] slots_top = new int[]{1};
private static final int[] slots_bottom = new int[]{1};
private static final int[] slots_sides = new int[]{0};

public static float Temp = 0;
public static float maxTemp = 300;
public static ItemStack[] slots = new ItemStack[2];
public static int smeltItem;
public static int furnaceSpeed = 100;
public static int cookTime = 0;;

public boolean isBurning(){
	return this.Temp > 0;
}

public int getsizeInventory(){
	return this.slots.length;
}

private boolean canSmelt(){
	if(this.slots[0] == null){
		return false;
	}else{
		ItemStack itemstack = FurnaceRecipes.smelting().getSmeltingResult(this.slots[0]);

		if(itemstack == null) return false;
		if(this.slots[1] == null) return true;
		if(!this.slots[1].isItemEqual(itemstack)) return false;

		int result = this.slots[1].stackSize + itemstack.stackSize;

		return (result <= getInventoryStackLimit() && result <= itemstack.getMaxStackSize());


	}
}

public void smeltItem(){
	if(this.canSmelt()){
		ItemStack itemstack = FurnaceRecipes.smelting().getSmeltingResult(this.slots[0]);

		if(this.slots[1] == null){
			this.slots[1] = itemstack.copy();
		}else if(this.slots[1].isItemEqual(itemstack)){
			this.slots[2].stackSize += itemstack.stackSize;
		}

		this.slots[0].stackSize--;

		if(this.slots[0].stackSize <= 0){
			this.slots[0] = null;
	}
}
}

public void updateEntity(){
	if(worldObj.getBlock(xCoord, yCoord-1, zCoord).equals(Blocks.lava)){
		if(this.Temp < this.maxTemp){
			this.Temp += 0.01F;
		}else if(!worldObj.getBlock(xCoord, yCoord-1, zCoord).equals(Blocks.lava)){
			if(this.Temp > 0){
				this.Temp--;
			}
		}
	}
	System.out.println(this.cookTime);

	boolean flag = this.Temp > 0;
	boolean flag1 = false;


	if(!this.worldObj.isRemote){
		if(this.Temp == 0 && this.canSmelt()){

			if(this.Temp > 0){
				flag1 = true;

				if(this.slots[0] != null){
					this.slots[1].stackSize--;

					if(this.slots[1].stackSize == 0){
						this.slots[1] = this.slots[1].getItem().getContainerItem(this.slots[1]);
					}
				}
			}
		}


		if(this.isBurning() && this.canSmelt()){
			this.cookTime++;
		}

			if(this.cookTime == this.furnaceSpeed){
				this.cookTime = 0;
				this.smeltItem();
				flag1 = true;
			}
			}else{
				this.cookTime = 0;


		if(flag != this.isBurning()){
			flag1 = true;
			LavaFurnace.updateSaltFurnaceBlockState(this.Temp > 0, this.worldObj, this.xCoord, this.yCoord, this.zCoord);
		}


			}

		if(flag1){
			this.markDirty();
	 }

}


@Override
public String getInventoryName() {
	return null;
}

@Override
public boolean hasCustomInventoryName() {
	return false;
}

@Override
public int getInventoryStackLimit() {
	return 64;
}

@Override
public boolean isUseableByPlayer(EntityPlayer var1) {
	return this.worldObj.getTileEntity(this.xCoord, this.yCoord, this.zCoord) != this ? false : var1.getDistanceSq((double)this.xCoord + 0.5D, (double)this.yCoord + 0.5D, (double)this.zCoord + 0.5D) <= 64D;
}

@Override
public void openInventory() {

}

@Override
public void closeInventory() {

}

@Override
public boolean isItemValidForSlot(int var1, ItemStack var2) {

	return var1 == 2 ? false : (true);
}

@Override
public int[] getAccessibleSlotsFromSide(int var1) {
	return var1 == 0 ? slots_bottom  : (var1 == 1 ? slots_top : slots_sides);
}

@Override
public boolean canInsertItem(int var1, ItemStack var2, int var3) {
	return this.isItemValidForSlot(var1, var2);
}

@Override
public boolean canExtractItem(int var1, ItemStack var2, int var3) {
	return var3 != 0 || var1 != 1 || var2.getItem() == Items.bucket;
}

public boolean isInvNameLocalized() {
	return false;
}

public String getInvName() {
	return "container.lava_furnace";
}

@Override
public int getSizeInventory() {
	return 0;
}

@Override
public ItemStack getStackInSlot(int var1) {
	return this.slots[var1];
}

@Override
public ItemStack decrStackSize(int var1, int var2) {
	if(this.slots[var1] != null){
		ItemStack itemstack;
		if(this.slots[var1].stackSize <= var2){
			itemstack = this.slots[var1];
			this.slots[var1] = null;
			return itemstack;
		}else{
			itemstack = this.slots[var1].splitStack(var2);
			if(this.slots[var1].stackSize == 0){
				this.slots[var1] = null;
			}
			return itemstack;
		}
	}
	return null;
}

@Override
public ItemStack getStackInSlotOnClosing(int var1) {
	if(this.slots[var1] != null){
		ItemStack itemstack = this.slots[var1];
		this.slots[var1] = null;
		return itemstack;
	}

	return null;
}

@Override
public void setInventorySlotContents(int var1, ItemStack var2) {
	this.slots[var1]= var2;

	if(var2 != null && var2.stackSize > this.getInventoryStackLimit()){
		var2.stackSize = this.getInventoryStackLimit();
	}
}

public int getCookProgressSacled(int i){
	return this.cookTime * i / this.furnaceSpeed;
}

public static boolean isItemFuel(ItemStack itemstack1) {
	return false;
}

}

 

 

 

Container:

 

 

package info.coremodding.realsmelting.gui.containers;

import info.coremodding.realsmelting.tileentities.TileEntityLavaFurnace;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.entity.player.InventoryPlayer;
import net.minecraft.inventory.Container;
import net.minecraft.inventory.Slot;
import net.minecraft.inventory.SlotFurnace;
import net.minecraft.item.ItemStack;
import net.minecraft.item.crafting.FurnaceRecipes;

public class ContainerLavaFurnace extends Container{

private TileEntityLavaFurnace lava;

public ContainerLavaFurnace(InventoryPlayer inventory, TileEntityLavaFurnace entity) {
	this.lava = entity;

	this.addSlotToContainer(new Slot(entity, 0, 56, 36));
	this.addSlotToContainer(new SlotFurnace(inventory.player, entity, 1, 116, 35));

	for(int i = 0; i < 3; i++){
		for(int j = 0; j < 9; j++){
			this.addSlotToContainer(new Slot(inventory, j + i * 9 + 9, 8 + j * 18, 84 + i * 18));
		}
	}

	for(int i = 0; i < 9; i++){
		this.addSlotToContainer(new Slot(inventory, i, 8 + i*18, 142));
	}
}

public ItemStack transferStackInSlot(EntityPlayer player, int slot0){
	ItemStack itemstack = null;
	Slot slot = (Slot)this.inventorySlots.get(slot0);
	if(slot != null && slot.getHasStack()){
		ItemStack itemstack1 = slot.getStack();
		itemstack = itemstack1.copy();

		if(slot0 == 2){
			if(!this.mergeItemStack(itemstack1, 3, 39, true)){
				return null;
			}
			slot.onSlotChange(itemstack1, itemstack);
		}else if(slot0 != 1 && slot0 != 0){
			if(FurnaceRecipes.smelting().getSmeltingResult(itemstack1) != null){
				if(!this.mergeItemStack(itemstack1, 0, 1, false)){
					return null;
				}
			}else if(TileEntityLavaFurnace.isItemFuel(itemstack1)){
				if(!this.mergeItemStack(itemstack1, 1, 2, false)){
					return null;
				}
			}else if(slot0 >= 3 && slot0 < 30){
				if(!this.mergeItemStack(itemstack1, 30, 39, false)){
					return null;
				}
			}else if(slot0 >= 30 && slot0 < 39){
				if(!this.mergeItemStack(itemstack1, 0, 1, false)){
					return null;
				}
			}
	}else if(!this.mergeItemStack(itemstack1, 3, 39, false)){
		return null;
	}
		if(itemstack1.stackSize == 0){
			slot.putStack((ItemStack)null);
		}else{
			slot.onSlotChanged();
		}

		if(itemstack1.stackSize == itemstack.stackSize){
			return null;
		}

		slot.onPickupFromSlot(player, itemstack1);
}
	return itemstack;
}

@Override
public boolean canInteractWith(EntityPlayer var1) {
	return this.lava.isUseableByPlayer(var1);
}

}

 

 

 

If you need any other code i will post it.

Bump

Look at this maybe?

 

 

@Override

public int getSizeInventory() {

return 0;

}