Set in California, but a Bank of Montreal building and a Canada Post box appear repeatedly. A Scotia Bank is visible in the last scene.
In the last scene, the Xenos attack in broad daylight after it's been established that light is lethal to them.
At the beginning, a camera is reflected in the door of Carnby's taxi.
When Reid inspects the contents of a box in her apartment, the shadow of a microphone can be seen on a pillar.
During the taxi chase at the beginning, the cab chasing Edward Carnby has no rear license plate in one shot, but gains one in the next shot.
One of the soldiers checks the pulse of a person lying on the ground, to see if the man is dead, with gloves on.
Agent Cheung dies in the cavern. When the team tries to find a way out of the tunnels, Agent Cheung raises her head just as Agent Barr walks past her.
When Burke runs to set off the bomb, exterior shots of the sole remaining soldier show darkness. When Carnby and Cedrac blast through the storm door outside the orphanage, it's daylight.
When the camera pans the dead soldiers in the darkness, a camera man, trying to be as still as he can, is standing in the darkness behind the light on the stand.
The bandage on Cedrac's arm, and the wound it covers, disappears, then reappears in her apartment.
After Carnby's cab is slammed into the wall and the villain backs up to ram the cab again, the camera shot shows the villain "throwing" the car into reverse. The car should already be in reverse, and should be shifted into drive to ram the other taxi.
Some police cars arrive at the orphanage "20 years ago" which is probably 1984 or 1985. They have three brake lights, a much more recent feature.
Tara Reid pronounces "Newfoundland" the way it is spelled ("NewFOUNDland"). The correct pronunciation is "NewFUNDland." Not a minor error if the viewer is Canadian.
The end credits list a "TTransportation Captain".
When Pinkerton slams Carnby's head against the ground during their battle in Chinatown, several inches are visible between the actor's head and the floor at the moment of impact.
A taxi rams Carnby's taxi, denting the hood and bumper. Seconds later, when they turn a corner, the taxi is undamaged.
When Ron is killed, he falls face down. When Edward Carnby and Aline Cedrac find him, he is on his back.
After the explosion at the end, Carnby and Cedrac exit the wooden structure near the orphanage. Earlier, when Carnby visited the orphanage to ask for the children's files, the structure wasn't there.
When Carnby yells to Burke and then fires a shot to shoot a "zombie" coming behind Burke, all bullets are made visible with light. You clearly see the shot completely missing the target and flying off way past her head and yet she acts like she was hit.
When the agent is showing Edward pictures of people that have disappeared he says that there are only 3 pictures, but, he is actually showing him 4 people.