Factual error: The aliens request a meeting with Dr. Marvin on the shores of the Chesapeake Bay, near Washington DC. However, the shot of him driving along the shore line shows mountains in the background, which don't exist in Eastern Maryland.
Continuity mistake: In the scene in which Hugh Marlowe and his fiancée are on the alien ship, speaking with the aliens, his arm is over her shoulder when shown from the front, but down and he is about a foot away from her when shown from behind. The shots alternate back and forth, front and behind, so that the mistake is fairly obvious.
Visible crew/equipment: During the final attack after Carol falls on the floor in a hallway in the Pentagon there's a shot of a crowd running towards the camera. The woman in the front has the shadow of the boom mike right on her forehead as she comes close to the camera screaming bloody murder.
Continuity mistake: At Carol's barbecue a soldier drives up in a jeep with a message for Dr. Marvin stating that they lost contact with rocket No. 11. Dr. Marvin holds the message and an envelope in his hand as they look up and see the rocket burning up in the atmosphere. He never puts the message back into the envelope but when he sits down the envelope is alone and the messages is gone.
Deliberate mistake: After the alien ship destroys the bomber, Dr. Marvin picks up the helmet and they all start to run back to the car. In order to save time in repositioning the camera they start out 50 feet behind where they just were in the previous sequence and run back to the exact same spot.