Tree: Depth-First Search - Binary Tree Postorder Traversal
Consider this iterative postorder traversal code snippet:
```python
def postorderTraversal(root):
result = []
stack = []
current = root
while current or stack:
while current:
stack.append(current)
current = current.left
node = stack.pop()
result.append(node.val)
current = node.right
return result
```
What is the subtle bug in this code?