Continuity mistake: When Stephanie shows her ants and rabbits to Number 5, the amount of cats on the shelf behind suddenly increases between shots.
Continuity mistake: Ben brings a jacket to Newton, who looks at him. A shot later he is looking at the opposite way.
Other mistake: Newton Crosby (the programmer for the robots) is at the Vista house and Stephany is in Astoria. More than 113 miles (2 hours) away and Stephany is calling to meet up with Crosby to talk about number 5. I don't think 2 hours passed before they met up. All this happens within 5 min. Not to mention how did Crosby get out to Vista house so fast?
Continuity mistake: At the beginning of the movie when the "Red Army" approaches, the tank has his barrel locked in a transportation brace. As soon as the tank fires back, the brace is gone.
Continuity mistake: In the scene right after Number 5 almost drives the snack wagon into the lake and the Nova guys are shooting him, you clearly see him get shot in the hydraulic pistons which control his head motions. Yet, in the next scene (in the truck) Number 5 comfortably moves his head.
Answer: There were always 5. You see the 5 during the demonstration. Then, before the lightning hits, we see 4 of them in line, but number 5 was still attached to the generator and not in line. When they go inside, there's a weird cut that makes it look like only 3 made the turn, but if you watch closely you can see there's 4 followed by number 5.
Bishop73