Continuity mistake: Just after Samuel and Alfred say that they are going to enlist, Samuel has a weird looking face and notice Alfreds shoulder right behind him, then Susanna takes a few steps up the stairs very sad and Samuel walks a couple of meters towards her and then she leaves and then it switches to Samuel with the same funny looking face and Alfred is again right behind him.
Continuity mistake: When Tristen arrives home after being gone for many years - he sits with his father, Col. William. The Colonel has a chalk board around his neck and he writes, "Am Happy." Tristen replies, "Me too." The next scene shows the "Am Happy" chalk board again, but the printing is different from when the Colonel wrote on it in the previous scene.
Trivia: When shooting the scene where Isabelle Two is shot and Tristan gets beat up by the policemen, the actor playing the policeman accidentally really hit Brad Pitt on the head with the baton, giving him a black eye. Shortly after that, they shot the tennis scene that's near the beginning, and Brad Pitt still has his black eye. They had to make up a couple of lines about his "shiner" to explain it.