Revealing mistake: The stick used to make Jack's dog levitate is sometimes visible, for example when Jack is alone on the mountain.
Revealing mistake: During the song "This is Halloween" at the beginning, when the bats fly in front of the moon, the strings that they are attached to are visible.
Answer: As seen in the movie, the Fog Juice made it difficult to see anything. Sally had hoped that with no way of seeing where he was going, she would be able to stop him from leaving Halloween Town and save his life.