Revealing mistake: The movie takes place over the course of several years, as large segments of time pass between scenes of Will and Jane meeting at the airport, and other encounters. Thus, it is rather jarring to notice that their twelve year old children, Ben and Claire, don't appear to age at all. Young adolescents usually change in appearance quite drastically as they mature.
Continuity mistake: One scene is captioned as being the end of the summer, and the kids are going back home for school. The next caption, about ten minutes after that scene, tells us that it is six months later, and also Christmas. But Christmas is just three months after the end of the summer, so either the caption is partially wrong, or the few intervening scenes carried us from September to the following June without even seeing the intervening holiday season, and then the caption was meant to jump us by an additional six months.