When Jenna is making Dawn's "Falling In Love" chocolate mousse pie, the sun is pouring in from the window behind her. The previous shot established it was already night.
When the Exhausted Mother and her son are walking up to the porch, the silly string is blue. When the son sprays her during the first close up of her face, the silly string is pink. When the camera switches to Jenna then comes back to the mother, the silly string is blue again.
When Oki comes into the diner to give Dawn the flowers, they are in the corner of the booth. In he next shot they're in front of them, and he clearly did not move them.
When Jenna goes to get her first ultrasound she is wearing a pale sky blue button-down shirt. But after the ultrasound, when she is in the doctors office, her shirt is a periwinkle blue, closely resembling a light lavender blue and does not appear to have buttons, but a V-shaped collar, but you can only see the full shirt very briefly at the beginning when they are both sitting in his office. Then, immediately after, when she's walking on the street, her shirt changes back to a pale sky blue color, but the buttons are harder to see. And when she's seated at the bus stop the buttons are very prominent and the color is slightly different. The color changes could be do to lighting, but more than likely they are different shirts.
During the movie, Jenna makes two lattice-top pies by laying all of the dough strips in one direction and then placing the dough strips for the other direction on top of the previous strips. Usually lattice-top pies are made with the dough strips for both directions woven together with each other. (That way they look better and bake more evenly.)
When Jenna has her ultrasound scan, the doctor tells her that it's a girl. The picture shows a boy.
Whenever Jenna deals with the stove or oven, whether she's putting the pies in, taking them out, or melting chocolate in the sauce pan, it's always off.
When the doctor is dropping Jenna off at work right after their first passionate kissing, Jenna has lipstick all over her face, but his is perfectly clean.
After the opening credits, Jenna can be seen absently mixing chocolate in a bowl. But the closeup view of the bowl shows it is cleaner on the sides than on the wider shot.
Assuming this movie takes place in the present, the nurse's uniform in the delivery room is a least twenty years out of date. Some modern hospitals still require nurses to wear all white, but they wear scrubs. Even the most die-hard facilities stopped wearing caps by the mid 1980's.
(at around 12 mins) During Jenna's first visit to Dr. Pomatter, he takes a blood sample but doesn't examine her. He says that she is 6 weeks pregnant, but there is no way he can determine this based on the blood work alone.
When Joe orders his lunch, he distinctly asks for two glasses of water with no ice to be brought to the table before the orange juice and the meal. Jenna brings two glasses of water, one orange juice, pie, and tomato. Joe complains about the ice, and the fact that Jenna did not bring salt and pepper to the table, but does not criticize the fact that Jenna brought the whole order together, rather than separately, as instructed.