Other mistake: After the Sanderson sisters are done sucking the life out of Emily Binx, they do a little dance thing after they become younger. When either Winifred or Mary swings past her, Emily's leg moves even though she's supposed to be dead. She's also playing with her hands and bobbing her head.
Other mistake: When Mary goes to save Winifred, there is no cord coming from the Hoover. However, next scene shows Dani, Allison and Billy grab a medium sized cord. Next scene then shows the cord much much much bigger.
Other mistake: In the first graveyard scene, when Allison swats a tree branch at Sarah, Sarah yells, "Ouch," and flies away but the branch never touched her.
Other mistake: At Allison's party after she gets Max and herself a drink, one shot shows her going to take a drink but in the next shot from behind she takes up the cup again and drinks.
Other mistake: Winnie could not have known that Max said, "It's just a bunch of hocus pocus," considering the three of them were still dead at that time. (They couldn't have been ghosts, because she didn't know who lit the black-flame candle.)
Other mistake: The same footage of Allison making the salt circle in Max's room is used both times we see her do it.
Answer: They were set up, beforehand, just in case. Winnifred knew about the spell, but she didn't memorize the incantation. So she needed the book to see how to cast it.
Phixius ★
If you pause the movie, you will see that the book opened up to the Black Flame Candle spell.