Program: 187568
Project: 187568
Buggy source:
public int addItem(org.bukkit.inventory.ItemStack itemAdd){
  if (itemAdd == null) {
    return 0;
  }
  itemAdd=itemAdd.clone();
  int itemID=itemAdd.getTypeId();
  int itemDuarbility=itemAdd.getDurability();
  int itemMaxStack=itemAdd.getMaxStackSize();
  for (int i=0; i < this.getSize(); i++) {
    ItemStack item=getItem(i);
    if (item != null) {
      if (getItem(i).id != itemID) {
        continue;
      }
 else       if (getItem(i).getData() != itemDuarbility) {
        continue;
      }
 else       if ((item.getEnchantments() != null && item.getEnchantments().size() > 0) || (itemAdd.getEnchantments() != null && itemAdd.getEnchantments().size() > 0)) {
        continue;
      }
 else       if (item.count >= itemMaxStack) {
        continue;
      }
 else       if (itemAdd.hasItemMeta()) {
        continue;
      }
      if (itemAdd.getAmount() >= itemMaxStack - item.count) {
        itemAdd.setAmount(itemAdd.getAmount() - (itemMaxStack - item.count));
        item.count=itemMaxStack;
      }
 else {
        item.count+=itemAdd.getAmount();
        itemAdd.setAmount(0);
        break;
      }
    }
  }
  for (int i=0; i < getSize(); i++) {
    if (itemAdd.getAmount() <= 0) {
      break;
    }
    if (getItem(i) == null) {
      if (itemAdd.getAmount() <= itemMaxStack) {
        setItem(i,CraftItemStack.asNMSCopy(itemAdd.clone()));
        itemAdd.setAmount(0);
        break;
      }
 else {
        org.bukkit.inventory.ItemStack itemStack=itemAdd.clone();
        itemStack.setAmount(itemStack.getMaxStackSize());
        setItem(i,CraftItemStack.asNMSCopy(itemAdd.clone()));
        itemAdd.setAmount(itemAdd.getAmount() - itemStack.getMaxStackSize());
      }
    }
  }
  update();
  return itemAdd.getAmount();
}

Fixed version (ground truth):
public int addItem(org.bukkit.inventory.ItemStack itemAdd){
  if (itemAdd == null) {
    return 0;
  }
  itemAdd=itemAdd.clone();
  for (int i=0; i < this.getSize(); i++) {
    CraftItemStack craftItem=CraftItemStack.asCraftMirror(getItem(i));
    if (ItemStackComparator.compareItem(itemAdd,craftItem)) {
      if (craftItem.getAmount() >= craftItem.getMaxStackSize()) {
        continue;
      }
      while (craftItem.getAmount() < craftItem.getMaxStackSize() && itemAdd.getAmount() > 0) {
        craftItem.setAmount(craftItem.getAmount() + 1);
        itemAdd.setAmount(itemAdd.getAmount() - 1);
      }
      if (itemAdd.getAmount() == 0) {
        break;
      }
    }
  }
  if (itemAdd.getAmount() > 0) {
    for (int i=0; i < this.getSize(); i++) {
      if (getItem(i) == null) {
        if (itemAdd.getAmount() <= itemAdd.getMaxStackSize()) {
          setItem(i,CraftItemStack.asNMSCopy(itemAdd.clone()));
          itemAdd.setAmount(0);
          break;
        }
 else {
          CraftItemStack itemStack=(CraftItemStack)itemAdd.clone();
          itemStack.setAmount(itemStack.getMaxStackSize());
          setItem(i,CraftItemStack.asNMSCopy(itemStack));
          itemAdd.setAmount(itemAdd.getAmount() - itemStack.getMaxStackSize());
        }
        if (itemAdd.getAmount() == 0) {
          break;
        }
      }
    }
  }
  return itemAdd.getAmount();
}