Junit testing missing tests

I'm writing tests for Junit to test a delete function that I wrote:

@Override
    public T delete(int index)  {
        if (index < 0 || index > this.size()) {
            throw new IndexOutOfBoundsException();
        } else if (isEmpty()) {
            throw new EmptyContainerException();
        } else {
            Node<T> current = front;
            if (index == 0) {
                front = current.next;
                current.prev = null;
                size--;
                return current.data;
            } else if (index == size - 1) {
                return remove();
            } else {
                current = traverse(current, index);
                Node<T> temp = current;
                current.prev.next = current.next;
                current.next.prev = current.prev;
                size--;
                return temp.data;
            }
        }
    }

This method is for a double linked list that has both a back and front node.

THE PROBLEM: Our college will run buggy code against the tests we write to determine whether we've written enough tests to catch bad code and exceptions.

I know 2 of the tests that they will run, but don't know what the error means.

• FAIL: MissingBackFieldRepairLogic

  Unable to find bug with DoubleLinkedList with missing back field repair logic
  
  

• FAIL: MissingNextNodeRepairLogic

  Unable to find bug with DoubleLinkedList with missing next node repair logic
  
  

These^ are 2 tests that I have not accounted for as I'm unable to understand what theses errors mean. Does anyone have any idea about what these errors might be?

And what kind of tests should I write to catch these errors?

Thanks -A desperate student