Revealing mistake: Right before the witches burst through the roof of Max's house, after they retrieve the book, you can see the charge used to set off the explosion in the window just below where they exit.
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.
Revealing mistake: As Mary flies toward the kids in the cemetery, the wires holding her are visible.
Continuity mistake: As Alison holds off the Sanderson sisters with the box of salt, some shots show her holding the salt up close to the Sandersons while other shots the box of salt Is further away from them.
Continuity mistake: When the witches figure out that the "Burning Rain of Death" was actually water, it showed that they were all wet from before they managed to get under cover. In the next shot, as they leave the house, they're all bone dry.
Continuity mistake: When the sisters are in "Satan's" house and meet the wife, her curlers change position.
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.