Continuity mistake: In the scene when the mom is making sandwiches she picks up a piece of meat and puts it on the sandwich in her hand but then when the shot changes she is taking that same piece of meat and putting it on the same sandwich, and you can see that she isn't taking another piece and putting on the sandwich because you can see that all she has on the sandwich in her hand is a piece of lettuce.
Continuity mistake: In the scene during the credits, the right shoulder strap on Marie's dress has fallen off her shoulder. Her body turns as she dances in the circle and her right shoulder is not visible momentarily. When it becomes visible again, her strap is up. No one touched her and she did not "shrug" it up.
Continuity mistake: The whole time Dan is in the bookstore and outside talking to Marie, he is wearing a blue undershirt. However on his way home from the bookstore, he runs a stop sign. When the cop comes up behind him you can see that his undershirt is now grey. When he gets home and walks through the door, his shirt has gone back to blue. So Dan's shirt changes a few times when everything is supposed to be in the same sequence of time and events.
Factual error: The license plates on Dan's car are very inaccurate fakes. The colors especially are way off. Certainly, the makers may use a fake sequence of characters the same way "555" phone numbers are, but the styling of the plate should look right. Real NJ plates are a pale yellow along the top 1/4 that fades to a very pale tan by halfway down. Dan's plate is a bright yellow (nearly orange) at the top and the rest is white. The numerals are also the wrong typeface. The mistake is made more obvious by the fact that real New Jersey plates are seen in the early portion of the film on other cars.
Continuity mistake: In the scene where Dan is in the bookstore, he has a jacket on when he walks in. And right before Marie arrives he is still wearing it. As soon as she arrives, the camera pans over to see her. Then it pans right back to Dan and his coat is gone. The coat does not show up again until the next scene when they go outside to talk and eat the muffin together.