Character mistake: Selena Gomez drives into town in a fully restored 1967 Pontiac Le Mans, which is a long box shaped car. However, when confronted with Bill Murray and Adam Driver, Adam Driver complements her on her 1968 Pontiac Le Mans, which is a completely redesigned shorter curved body style. The two look nothing alike.
Character mistake: Detective Spitzer went into Harper's dark/unlit house, saw his partner Greg on the floor, and then saw Alec standing in the kitchen. Without identifying himself, telling Alec to drop his weapon (pistol pointed toward the floor), or bothering to determine what happened, Detective Spitzky shoots Alec. (01:27:08)
Character mistake: When Detective Mike walked past 13-year-old Andy and his friends (depicted as neighborhood children or young teens), he said, "Little f-ing millennials." Millennials were born between 1981 - 1996, so their current ages would be 26 - 41 (or about three years younger when the movie was made). Andy and his friends would fall under Generation Z ("Zoomers"), born between 1997 - 2012 (current ages 10 - 25 or 3 years younger when the movie was made). (00:25:52)
Character mistake: When they're making the protection circle they make a star of David and not a pentagram, 6 points in a star vs 5 points.
Suggested correction: I don't think there was anything in the movie that dictated a pentagram had to be used. In the book of protection rituals they were using, the particular ritual showed a picture of the star of David they needed to create, so that's what they used (plus salt).
To add to the correction, what the mistake refers to as "Star of David" is a hexagram and the use of hexagrams in the occult are not uncommon. You can find salt protection circles that use the hexagram.
Character mistake: Mia's Sea-Monkey, in a vertical position, was shown sinking to the bottom the jar. It isn't clear if the Sea-Monkey was dead or if Mia was referring to it when she said, "She's gonna freeze to death, Aiden", but Mia should have known how to properly care for her Sea-Monkey (or refer to the instructions) and, realizing the room was too cold, should have done something - such as wrapped a towel/blanket around the jar and/or moved the jar to a warmer location (e.g, near the fireplace). (01:22:33)
Character mistake: Reina asked Cory if they could "do that thing" because it was helpful for her medical school training. Reina then put a raw slab of steak/meat across his forehead and made slits in the meat as well as jabbed at it. A medical student studying to become a brain surgeon should know about and practice the safe handling of raw meat. (00:23:10)
Character mistake: There are some musicians at different times in the movie. One of them is playing a moraharpa (traditional Swedish instrument, a predecessor of the nyckelharpa), but holding and playing it incorrectly. (00:38:37 - 01:39:45)