Character mistake: Given Billy still remembered and recognized him it seems strange Kate doesn't when she mistakes Daffy for Gizmo.
Suggested correction: Billy spent significantly more time bonding with Gizmo in the first movie - days and days. By contrast Kate had very little actual time with him.
Character mistake: Archimedes says he told Arthur not to pull the sword out of the stone. He didn't, he pointed it out in the churchyard to him instead.
Suggested correction: Archimedes was trying to be helpful when he pointed out the sword so Merlin could take it to the tournament. But after it became a big issue with the rays from heaven and the legend, Archimedes thought he might get in trouble so tried to act innocent by fibbing. It was deliberate and in character, not a character mistake.
Character mistake: When Darcy walks into the police station the chief calls her Joan, the lead actress' real name, by mistake.
Character mistake: The Mother gave the Man (as Panda the cat) a bowl of vanilla ice cream with whipped cream and apparently some chocolate syrup. Chocolate is toxic to cats and most cats are lactose-intolerant. A mature/adult person should be aware that "people food", in general, is not healthy for cats and certain foods (chocolate and cow's milk) are particularly bad and possibly fatal. (00:27:27)
Suggested correction: This situation is actually more complicated in that the mother was seeing the man (her dead grandson), so wasn't really giving Panda the cat the ice cream. What can't be known is whether Panda as the man could eat ice cream with chocolate syrup and not get sick (e.g, vomit) after he turned back into a cat the same day.
Character mistake: When Inspector Owen calls the village police station the call is answered by a sergeant but at the end of the call Inspector Owen refers to him as Inspector. (00:10:00 - 00:11:00)
Character mistake: There are signs out front as the Spenglers arrive at the dirt farm. One of the signs is a call back to a scene in Ghostbusters 1984. In the scene, Ray quotes the book of Revelation: "And I looked, and he opened the sixth seal, and lo, there was a great earthquake. And the sun turned black as sack cloth, and the moon turned blood." When Finn Wolfhard reads it he misquotes the line saying "The sun became as black as cloth."