Continuity mistake: Towards the end of the film Nicole Kidman is having her wedding photos taken. The photographer poses her for the shot, but when he actually takes the photo, the background has changed. When he poses her, there are just trees and bushes behind her, but when he takes the photo, the trees and bushes are gone and people are walking around.
Continuity mistake: In the bedroom scene where Joseph discusses the boy's letter with Anna, we see Anna putting on her earrings, interestingly enough, her earrings change in each shot. First we see her putting on her 'left' earring while she is still standing by the bureau, then she sits down on the bed and puts on her 'left' earring again, then she stands up - and in the next shot we see her at the buffet where she is suddenly already wearing her 'right' earring, but miraculously missing her 'left' earring. (00:22:00 - 00:23:25)
Answer: I think that 'what sets Anna off' at the ending was Sean's letter to her...where in the very last line, he says, "I guess we'll meet in another life." This shows that Sean really was reincarnated, but he so didn't want Anna to learn about the the affair with Clifford's wife, that he (Sean) decided to let go of Anna, possibly for the time being until he grows up, leaving room for a future sequel.