Consider this modified Morris inorder traversal code snippet. Which line contains a subtle bug that can cause infinite loops or incorrect output on some trees?

Consider this modified Morris inorder traversal code snippet. Which line contains a subtle bug that can cause infinite loops or incorrect output on some trees?

def inorderTraversal(root):
    result = []
    current = root
    while current:
        if not current.left:
            result.append(current.val)
            current = current.right
        else:
            predecessor = current.left
            while predecessor.right and predecessor.right != current:
                predecessor = predecessor.right
            if not predecessor.right:
                predecessor.right = current  # create thread
                current = current.left
            else:
                # BUG: missing line to remove thread
                result.append(current.val)
                current = current.right
    return result