Here's why:
* Unintentional Nature: In the scene, Haley is carrying a very heavy metal costume. It's likely he stumbled on the uneven ground, which was common on movie sets at the time.
* No Dialogue Mention: The scene doesn't suggest any intent on the Tin Man's part. He simply falls, and the other characters react accordingly.
* Humor & Continuity: The slip adds humor to the scene, but it also serves the story's continuity. It emphasizes the Tin Man's lack of mobility and reminds us of his vulnerability.
It's worth noting that the scene was filmed in a studio setting with a painted backdrop, so the "yellow brick road" was not a real surface. This could also have contributed to Haley's stumble.
While the slip may not be intentional, it adds a layer of realism and humor to the scene, making it more memorable.