When Lassie decides to leave the elderly couple, the man opens and closes the gate for her to leave. The camera angle then changes to show the gate still open, and then the camera returns to the original angle to show the gate closed again.
The first time Lassie escapes from the kennel, she digs a hole in the dirt to sneak under the fence. However, after all her digging, her paws and chest hair are still pure white and clean.
While speaking about whether to keep Lassie or not it is obvious that the large hearth behind the elderly couple is a drop screen. Shadows from the lights show behind the couple from the viewer's left to right, but the shadows on the hearth shine from the opposite direction.
In an early scene in the movie, the shadows are the same when Joe is going to school in the morning as when he's coming home that afternoon.