Oleg refers to the upper stage of the space shuttle. There is no "upper stage". There is the orbiter itself, the huge external tank that holds the fuel for the main engines, and the two solid fuel rocket boosters.
While Gabriel is using a payphone to call Martha's mother, he presses a single button on the keypad, and all 12 buttons depress below the face plate. Either the face plate itself is loose or the keypad below is detached from the face plate. In either case, he wouldn't be able to make a phone call using the keypad, because it's clearly broken.
When Aderholt pulls Beeman into The Vault to inform him of Gaad's death, the light is off before they enter. After Beeman closes the door, the light turns on.
When the agent Aderholt with his colleague monitor the mail robot, there is a IBM PS/2 on the desk in the corner of the room. The PS/2 has been launched in 1987, while the action take place before the death of Andropov which occurred in 1984.
The recorder is opened to reveal a miniature disk drive. A drive of this size would not be invented until years later.