Character mistake: When Jimmy explains to the kids of the town where the aliens took their parents, he mentions the origin of the aliens being somewhere in the Orion Star System, 3 million light years away. The Orion system is way closer than that. It is actually less than 1400 light years away from Earth. 3 million light years away would place them further than the Andromeda Galaxy.
Suggested correction: This shouldn't be a character mistake. A genius like Jimmy would know something like this. It's basic astronomy.
That's exactly why it's a character mistake. A character mistake is when a character does or says something that they shouldn't based on who/what they are suppose to be, or something a character wrongly states as fact when they should know better.
Character mistake: The first law class scene involves Reese Witherspoon getting kicked out of the room for being unprepared to answer questions regarding Gordon v. Steele, a very basic Civil Procedure case. The scene closes with the professor asking Selma Blair about the result in Gordon v. Steele. She responds that the court held that there was not diversity jurisdiction. The professor replies that Selma is correct. But, this is entirely the wrong answer. The court in that case held that there was diversity jurisdiction. Merely getting a case wrong would seem trivial, but again this is literally the first case any law student studies in Civil Procedure, a first year class. How could a screenwriter get this basic case completely wrong?
Character mistake: At the FBI facility, Agent 11's handler buckles the collar on the dog's neck. Later, when Gordon has put the collar on and tries to get it off, he's trying to pry it off. He just buckled it on not two minutes earlier. He's stupid, but not that stupid.
Character mistake: When Rianna introduces Marvin to the animals at the shelter, she says, "We have macaws," and then shows him Cosmo, saying, "My friend Cosmo is a parrot." This does not make sense because "parrot" is a broad group of several bird species, including both Cosmo and the macaws. Cosmo appears to be a type of Conure.
Character mistake: The acronym USGS (United States Geological Survey) is stated correctly at the beginning of the film, however, upon arriving at the military outpost, Dr Block says "United States Geological Society", not 'Survey'.
Character mistake: Joe Dirt mistakenly refers to his 1967 Plymouth GTX as a Road Runner while talking to Jill at the carnival.
Character mistake: When Mike pulls out the first Scare Report, the word "yield" is spelled wrong.
Character mistake: When Ramathorn and Farva pullover the Plymouth Voyager, Ramathorn radios in and says they have a Plymouth Voyager, Vermont plates Alpha, God, Mustard, 1, niner, decade, china. When Farva gets out of the car to call the driver a chicken f**ker, you can see the licence plate, Vermont plate '1 9415'. Ramathorn radioed in the wrong licence plate number.