Paul Frees is the apparent "spokesperson" for the aliens. His voice is heard throughout the movie. He's also the voice visitors hear in "The Haunted Mansion" rides at the Disney parks. See more...
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Continuity: 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.
Continuity: 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.