Continuity mistake: When the child serves the ice cream there is a difference in the size of the ice cream he serves and the spoonful he eats.
Other mistake: In the scene where an unemployed Ted Kramer character is in the offices of another advertising firm negotiating with two executives for a job, he tells them he will take a pay cut if they hire him but that this is a one-time offer. The executives are reluctant to make a decision because supposedly, it is Friday at 4:30 p.m., the weekend before Christmas. However, the shot through the window behind where Ted is sitting shows a bright and sunny New York City, probably at high noon since there are no building shadows. If it were really 4:30 p.m., the city would obviously be much darker, particularly in late December. Also, given the greenery that can be seen, it is probably not even winter.
Continuity mistake: In a scene in the restaurant, the rose on the table in the vase goes from a small rosebud to a fully blown rose and the petals are ready to fall off.
Visible crew/equipment: In the scene where Ted and Margaret are washing dishes, a microphone is seen coming up from the floor.