The experimenters claim there are 80 employees in the building, yet the parking lot only reveals 37 parked cars. Clearly everyone was not carpooling when going through the security checkpoint.
The building supposedly became a giant defacto Faraday cage, but they are able to pick up a local radio station from inside the building. But nothing supports that the metal around the building is also acting as a Faraday cage. In many outside shots the of building you can see a tower on the roof with 3 Sector antennas positioned around it, this is likely a cell tower and due to the remote location likely the only one providing service to the area. With the level of expertise shown by the perpetrators it would be simple to disable this tower during the lockdown. It is also easily more probable a cell jammer or jammers could have been activated in or near the building.
In the closing scene, the camera pans back to reveal a wall of monitors displaying CCTV footage of other Belko locations where employees are being subjected to the same experiment. However, upon closer inspection, the images are duplicates and triplicates.
Whenever the film cuts to the control panel's list of names, several are listed in "First name, Last name" format instead of the standard "Last name, First name" format used for all of the close-up shots. Additionally, some are misspelled, such as "Stve".
When Dany and Roberto are trapped above the moving elevator, it shows Barry going to the top floor. While this is convenient for the scene, it doesn't make sense for Barry to be heading to an empty floor immediately following orders to kill as many people as possible. Furthermore, in the very next scene he is suddenly on the first floor.
At the end of the film, Mike Milch kills the guards and experimenters by detonating the explosive chips that he planted on them; the very chips that were taken from the blown out skulls of his colleagues who died precisely because the chips had already detonated in their heads. Furthermore, the experimenters were able to track the conversations and movements of all of the employees via extensive audio and visual surveillance throughout the entire building; so they would have already known that the employees were attempting to extract the chips from their dead colleagues and that Mike would have some with him when he is brought into the hangar.
In the very first scenes, one office character reads the company profile out loud in dutch although he mispronounces every word.