Continuity mistake: After Shosanna switches projectors on the visual cue from the film, the movie cuts to Marcel standing behind the screen smoking a cigarette. The camera pulls back from behind him showing a large billow of smoke rising from his face and he lifts his right arm up to his cigarette. But then the camera cuts to an overhead view, and suddenly his arm is down and the smoke is gone.
Continuity mistake: When Donowitz and Ulmer walk into the theater, they see Joseph Goebbels and his mistress in the box above them, to their right. They would be looking to their right to watch the movie. Later, during the movie, Goebbels, Hitler, the mistress and Zoller are looking to their left to watch the movie, so they would have to be in the box on the opposite side of the theater, where Martin Bormann was shown.
Continuity mistake: When Hans Landa first approaches Bridget von Hammersmark in the theater, he is shown to be walking right up to her, as evidenced by the shot of his right foot being shown to be mere inches away from her left leg. When the shot changes to a medium shot of von Hammersmark saying "Colonel Landa, it's been years," Landa is suddenly out of shot, whereas the proximity of his feet to von Hammersmark's suggests he should have been visible in the medium shot. (01:51:10)
Continuity mistake: When Shoshanna meets Zoller, there's a strand of hair falling by her left cheek which appears or disappears depending on the angle focusing.
Chosen answer: It is explained shortly afterwards that a real German would hold up his thumb, index and middle fingers to indicate three. Since, the major already suspects the Captain of being a spy, holding up his index, middle and ring fingers to indicate three confirms this.
Xofer