When Colonel Weber comes by helicopter to take Louise, ha said Gavisti as the answer for Sanskrit word for war. Louise said it means desire for more cows. Actually, Sanskrit word for war is Yuddha. Gavishti means battle or desire for fighting, so it is close.
Near the end of the movie, when Louise has locked herself in the clean room, calling the Chinese General on the satellite phone, Agent Harper has his gun aimed at the window and tells her she is committing treason. Treason only applies with enemies of the U.S.. Although this is a fictional movie, there was no mention of the U.S. being at war with China.
Colonel Weber's badges on his Army Combat Uniform (ACU) disappear and reappear. Most notably, there is a scene where his Combat Infantryman Badge (CIB) is completely absent but reappears later in the movie.
When Louise and Ian enter the spacecraft the second time it's after 2 am: there's the predawn light when Louise is getting ready, as seen when the spacecraft is shown but when she exits the tent it's still pitch black outside.
When Louise goes to the college the day after the arrival to teach, the entire building is deserted. No students, no staff. And yet the cleaners had come in, cleaned and neatly arranged all the furniture as per usual.
In the main landings chart the Australian contact point seems to be in the Indian Ocean off Geralton in Western Australia. In a later scene there is a brief shot of an alien spacecraft with central Sydney in the distance across water. Sydney is on Australia's east coast.
A map on a TV screen shows one of the locations of the alien spacecraft landings as DEVON, UK. The spot marking the landing site is actually positioned over Wales on the screen. Devon is actually much further south.
In the beginning of the movie, Dr Banks says that Portuguese sounds 'different from the other Romance languages.' In actuality, Romanian is the romance language that is significantly different to the others. Portuguese is quite similar to several others.
When a Pakistani soldier is seen on the monitor, his uniform's color and shape are not those used by any of the forces of Pakistan. The ribbons on his left chest and the badge on his right chest do not belong to any forces of Pakistan. However, his formation sign on his upper left arm looks like one used by the Joint Staff.
Early in the film when Louise is watching news on the laptop in her office, one of the news broadcasts reports that the the Bureau of Alcohol, Tobacoo, and Firearms (ATF) has "put a temporary ban on new gun licenses, forcing many independent suppliers...to close their doors to the public". The ATF is a federal agency, but there is no such thing as federal firearms licensing for gun purchases as of the release of this film. The ATF issues licenses for the sale of firearms, but a ban on new licenses would have no impact on existing, licensed dealers.
The scene were Louise Banks and Ian Donnelly arrive by helicopter shows a panoramic view of the alien landing sight and human camp. As the scene progresses we get a stunning picture from the air. The campsite has what looks like 4 large lighting trees with shadows from the morning/afternoon sun projecting onto the ground over twice the lighting trees length. The alien ship on the other hand as no shadow apparent. Even though there is mist/fog on the ground below it, there should be some form of shadow visible from such a large object with the sun at that low level.
Louise is told to translate some overheard Mandarin and is momentarily confused at hearing the Mandarin word for "suits" before realizing that it is a reference to Mahjong. In actual Mandarin, the word for "suit" that refers to clothing and the word for "suit" that refers to a classification of cards in a card game are not the same word. If the speakers had really been using the word "suit" in the card-game sense, Louise might have initially translated it as "flower-color" and been confused, but the only way she could have translated it initially as "suit" would be if she already knew of the card-game meaning. However, this is not a "Factual Mistake". There is no indication in the movie that Louise ever thought of "suit" in the clothing-sense. There are indeed suits (in the card-game sense) in Mahjong, but Mahjong is not a card game; it is played with tiles. In other words, Louise may have translated it as "suit" in the card-game meaning without realizing yet that the referenced game was Mahjong; and that (as well as the fact that neither card-games nor Mahjong are directly linked to the topic of the rest of the Mandarin conversation) may be where Louise's initial confusion arose from.
The language displayed for shots of Pakistan and Pakistani news channel are in Arabic. Whereas the main and dominant literary languages used in Pakistan are Urdu and English. Arabic is spoken by barely 1% of the population and is never used in news channels.
When Banks and Donnelly are arriving at the Montana site, they are shown traveling in a CH-47 Chinook, a tandem rotor helicopter. However, as they are circling for landing, the shadow of their helicopter only has one rotor.
Actually, the CH-47 Chinook seen in those shots is a different helicopter landing at the base as the same time they are and seen from their point of view. The helicopter they are traveling in has one rotor.
Louise Banks tells Colonel Weber that "kangaroo" was in fact an Australian aboriginal word meaning "I don't know what that is." This is not true. It is an urban legend, which has been debunked since the 1970s. However, in the next scene she tells Ian Donnelly that she knew it was not a true story, and was only bringing it up as a convenient instructional metaphor.
The Chinese general speaks strange Chinese with strong Cantonese accent. This can't be real in modern Chinese military, or his colleagues and other soldiers wouldn't even understand him most of the time.
The movie treats Top Secret security clearances properly at first by taking away Ian's and Louis' phones, but the rest of the staff would also be under such restrictions. There is no way that Captain Conspiracy would have had access to a laptop with WiFi, let alone be allowed to access the sites he was shown to be watching.
The team ascends to the spacecraft on a platform that is elevated with a scissors mechanism. A wide shot shows the platform half-way between ground and underside of the spacecraft as well as how much the scissors are being unfolded at this stage. Thus when coming close to the underside, the scissors should be even more unfolded. However, a brief shot shows they are less unfolded when the team arrives at the underside.
The goof items below may give away important plot points.
Crew or equipment visible
Towards the end of the movie, when Louise comes back from having entered the shell on her own, Ian approaches her with a blanket which he throws around her shoulders. As he does, the blanket momentarily gets caught on some filming equipment, possibly a mic, before getting pulled free and draped around Louise.
When the US team decides they want to share their information, the CIA rep (Mike Stuhlbarg) says, "All we can do is shout at our allies at the border", etc, he is incorrect. Gen. Shang calls a "news conference" to say China is "standing down", then why couldn't we have done the same thing? In other words, we could have "broadcast wide", like the Russian scientist had. This is inherently a plot hole.