Visible crew/equipment: After the robot hears the Chuckle Brothers have found another way out (when they put up the fake wall to hide themselves from the robot) look carefully at the robot's head as it walks close to the camera, you can quickly see the cameraman in a blue shirt before it disappears behind the wall.
Revealing mistake: When the robot refuses to let Paul and Barry through, they attempt to go under his arms but they are thrown aside. It is obvious dummies are used, because of their floppy features and blank faces.