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."
Character mistake: They keep calling the vehicle a milk truck but it says "ice cream" on it. (00:23:55)
Character mistake: David Dastmalchian is the Polka-Dot Man, hyphenated in the comics, and in the end credits of the movie. Then again, when Waller threatens to kill everyone off, the name of the character appears in print as Polka Dot Man, no hyphen. (01:49:40)
Character mistake: When Joan is searching the web looking for information on what monkeys eat, there is a section on the screen that reads "The diet of monkeys varies form fruits, leaves." Obviously the correct word for this sentence should be "from" not "form." (00:36:21)
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: Eric says he found the little mermaid sculpture "on the coast of Cartagena" - as the subtitles confirm - but he mispronounces it as Cartageña.