Other mistake: When Jack first arrives in Christmas Land he doesn't know what is falling from the sky (snow) but he somehow knows that the children are throwing snowballs.
Continuity mistake: After Jack is blown out of the sky and is lying in the graveyard, he has several grey smudges on his face from the explosion. But in the very next shot, a bird's eye view of Jack lying across the angel, the smudges have disappeared. (00:59:10)
Continuity mistake: When Jack is having his Santa joyride, the army is called in to shoot him down. There is one sweeping shot of the giant cannons rising and firing at him. If you look at the bottom of the screen, you can see two small soldiers as the guns are rising, but as they fire, the soldiers blink out of existence. (00:58:30)
Continuity mistake: When the mayor gets into his car after he hears that Jack has been killed, his hat has to bend for it to fit under the roof. But a couple of scenes later, when Oogie Boogie hears the news, you see the mayor's hat; and it is short enough for it to fit without bending.
Continuity mistake: After Sally escapes from the Evil Scientist, she joins the crowd going into the town hall to hear about Jack's experience in Christmas Town. After the witch flies past Sally, watch the Mayor's loudspeaker car. The loudspeaker changes position about three times.
Continuity mistake: Throughout the movie, there have always been four vampires, but near the end, at the vampire hockey shot, there are actually five there. Three on the ice, and two behind the wall. They put the tall weird vampire out twice. (01:09:30)
Continuity mistake: When Sally is stitching herself up after her escape through the window, she puts the needle behind her ear when she is finished. The thread on the needle disappears just before she puts the needle behind her ear.
Other mistake: When Jack is handing out presents and the first house he goes to a kid catches him, and he hands him a present. Well you can see the bag of presents right next to the kid, but he never takes it back up, yet has it again later in the movie.
Continuity mistake: When Jack is telling Sally that he wants her to make his Sandy Claws outfit, he shows her a picture of him in a Sandy Claws outfit. We get a very clear look at the picture. However, when he is telling her which colors go where, you can see writing on the picture that was not there before.
Continuity mistake: When Sally delivers her care package for Jack, he is writing on his chalkboard. He scratches his head with the chalk in his hand. As he pulls his hand away, the chalk disappears mid-shot.
Continuity mistake: When Sally is making some food for Jack while he's trying to solve Christmas the scientific way, she shoves a cork onto a bottle. The cork is shoved in far enough that you'd need a corkscrew to pull it out. When she places the bottle into the basket, the cork isn't visible at all. When Jack gets the basket, he is able to open it because the cork is out enough for someone to pull at it.
Character mistake: At the beginning Sally follows Jack at the end of the Halloween celebration where he sings "Jack's Lament." She then goes off to gather more deadly nightshade. Look at Sally when she sits down. It is in front of a stone labeled "witch hazel" and "deadly nightshade" is seen next to it on the right. Yet, Sally pulls two pieces of witch hazel out of the ground, before a quick cut back to the lab where we see her putting it in a jar marked "nightshade".
Revealing mistake: The stick used to make Jack's dog levitate is sometimes visible, for example when Jack is alone on the mountain.
Continuity mistake: Sally lowers her care basket for Jack from her room on a string. When she jumps and in all later shots showing the basket, the string has disappeared.
Continuity mistake: In Christmas Town, while Jack is singing What's This, he grabs an umbrella from the snowman. Then he climbs on to one of the roofs and he leaves it in the snow. In the house he doesn't have it, but outside he has it again. (00:16:10)
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.