Continuity mistake: When Cecilia and Robbie are having sex against the bookcase, a close-up reveals she slips her left foot out of her shoe. But when Briony walks into the room and the two lovers are shown motionless, we see Cecilia still has both shoes on. (00:36:30)
Continuity mistake: When Briony sees Cecilia coming out of the fountain (from Briony's perspective), Cecilia steps out of the water with her right foot first. When we see the same scene afterwards, from Cecilia an Robbie's perspective, Cecilia steps with her left foot first.
Continuity mistake: In the love making scene in the library, Robbie pulls down the strap of Cecilia's dress and she pulls her arm out. In the next shot it is up and over her shoulder again.
Continuity mistake: When Briony walks in on Robbie and Celia in the library, the door is slightly ajar (since it's the lamp in the room that catches her attention). But when the scene is replayed from Robbie and Celia's perspective, Robbie shuts the door completely.
Continuity mistake: In the scene by the fountain from Briony's point of view, when Cecilia takes her top off she pulls the strap of her undergarment twice. However, from Robbie's point of view, she doesn't pull the straps at all.
Continuity mistake: In the first enactment of the fountain scene from Briony's viewpoint, the twins have just left her room and can be heard yelling downstairs for Celia's permission to go swimming. In an immediate, steady shot she goes to the window and witnesses the fountain scene already midway through. As we see in the next scene, from Celia's perspective, the twins run by asking to go swimming and she lingers in the piano room for several minutes and takes even a few more minutes chatting and walking with Robbie before they even begin the fountain scene.
Answer: Briony was just told by her superior to hold the French soldier's hand because he was dying. The French soldier was delirious and most likely confused her with another English girl whom he knew in the past. Since he was dying, Briony decided to play along and pretended that she was that girl. While this exchange is happening, though, Briony experiences something else as well. She sees the soldier as being like Robbie which is why I think she has such a strong reaction to him. When she tells the soldier "I love you" it is like her speaking to Robbie.