William's drawing from the middle of the movie is a different one from the one showed at the end of the movie. You can clearly see that drawn William on the first drawing is more sad than the William in the other drawing.
When snapping a picture of the newborn baby a flash can clearly be seen in the image, though no external flash is visible nor the internal flash from the Nikon camera.
When William is piloting the boat at the end of the movie, his hair changes style multiple times. At first it has a part in the bangs, then the part goes away, then the part returns exactly where it was before.
When Emilia and William are at the diner and Emilia asks the woman with the baby at another table how old the baby is, the woman answers, "Seven weeks; December 26." So the scene takes place in mid-February. When Emilia and William are shown leaving the diner in the next scene, the street trees in the background have mature green leaves, typical of summer. Deciduous street trees in New York City have no leaves in mid-February.
When William is skating he keeps calling the rink enclosure the wall. Anybody who grew up in the northern US calls it the boards.