Factual error: The baby is expanded to approximately fifty times his height, width and girth but it's explicitly stated that he retains his orginal mass. This means he will have the same volume as one hundred and twenty five thousand babies of his original size, but will weigh the same as before he was enlarged! That child is going to weigh much less than the volume of air he displaces. He's going to float away.
Continuity mistake: While Nick and his father are driving home with the giant baby behind (an obvious green screen projection), the baby breaks a tube from the car and throws it to the floor. Nick bends down to pick up a prop located offscreen, to add up for realism, but the problem is that he bends down before the baby throws it, so the whole trick is revealed.
Continuity mistake: When the mother and daughter are about to get inside the cab, two joggers pass by in the background. There's a shot of the father and son chatting for a long while, and when the angle goes back to the women, the joggers are on the same spot as before, instead of several meters away.