Goofs

Sam can't get on the ferry because Andrew lost her passport, but when he bought the ticket the previous day, the sign said "no passport required."

When Samantha asks Andrew about his son he states that he will be 6 yrs old tomorrow when his birthday is actually that very day because the night before he called his son to wish him a happy birthday and he said he was calling then because he wouldn't be able to call tomorrow.

When Kaulder finds the dead girl laying on the ground, and he covers her with his jacket, her legs are straight and uncrossed. When he walks back by her, one leg is now crossed over the other.

After the creature moves away from the gas station the TV screen is visible through the window despite the girl unplugs it seconds before.

A cephalopod mollusk (or even a vertebrate animal) of that giant size would have no ability to walk or move on land. It would be crushed by it's own massive weight. Since the creature is of extraterrestrial origin, however, its alien anatomy might have certain adaptations that allow it to walk on land without being crushed by its own weight.

Toward the beginning of the film when Sam and Andrew are first attempting to purchase ferry ride tickets to the U.S., the worker informs them that tickets currently cost 4,000 Colones due to high demand. A Colón is the currency found in Costa Rica and 4,000 colones is equivalent to about 8 U.S. Dollars.

When the two characters arrive at the deserted town on the US side of the border, and the camera pans the town and focuses on the evacuation side, you can see a pickup truck driving down the road. (This is likely the result of using open sets for filming, which allowed for the shooting areas local population to be accidentally captured in a shot.)

As the couple arrives at the town and the camera travels across the town and over the evacuation sign, right after the goof with the pickup-truck seen at the distance, they look at the town and a person can be seen walking away from the white car in the distance. Another result of shooting in open locations.

Small arms are apparently useless against the creature. Yet everybody crossing the Infected Zone or responding to an outbreak carries small arms.

However small arms are useful against humans, many humans would venture into the Infected Zone in search of possible exotic items and/or species. They may also lead unlawful expeditions into the zone for trill-seeking tourists. These trespassers might be armed and not want to be taken into custody and might respond with force.

There are no Mesoamerican pyramids on the border of Mexico and the United States.

The Pre-Columbian pyramid the two protagonists climb is located in a jungle setting. From the top of this pyramid they are able to view the border between Texas and Mexico, which is nowhere near any jungle.

At the start of the film, in "San Jose, Central America" we see a sign showing "US Border 200km". The shortest distance between the Central American border (Mexico/Guatemala) and the US Border is approximately 1,700km.

The scene where Samantha is about to board the ferry is located in Cancun on the southern part of Mexico but the map showing the infected zone is close to the border with the United States on the north.

Before Andrew buys train tickets, he looks at the giant wall map of the Infected Zone. All the train line stops have British names like Southhampton Central, Salisbury and Portsmouth all in their respective UK locations.

It absolutely beggars belief that anyone who has ever traveled internationally would allow someone who they just met a few days ago to hold onto their passport for them for any amount of time, let alone overnight. (Likewise, it's unbelievable that anyone would be willing to hold onto someone else's passport for them.)

There is never any apparent use for (or explanation of) the gas masks that people carry around and sometimes wear.

The creatures come from somewhere else in the solar system, perhaps Jupiter's moon Europa. But they are quite at home in Earth's gravity, atmosphere and normal temperature ranges.

They also must be feeding on terrestrial life, even though their biochemistry should be alien.

The USA is at war with the creatures, which hide there and lay eggs on trees. All they do is drop bombs. No use of napalm or defoliants, both of which would be highly effective.

Nor do they try poisoning the river where the creatures live. If they are alien, they would be vulnerable to something that didn't hurt Earth life.