Continuity mistake: After he eats the entire box of fudge bars, Adam has chocolate smeared all over his face. The smudges change shape and intensity several times between takes. (00:39:00)
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.
Revealing 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.
Revealing mistake: When Mandy falls inside the Porsche, her stunt double is noticeable because she is way older than her.
Continuity mistake: When Nick and his dad walk to the kitchen to see what happened with the baby, Nick leaves the newspaper sideways on the table. He and his dad run back screaming and hide behind the couch. Angle changes and the newspaper is now placed perfectly straight.
Continuity mistake: Seconds before Adam falls to the floor, Nick and Mandy show all the stuff the kid has in his pocket, but as he falls down, there's a wide look inside it and it's empty.
Continuity mistake: After Wayne calls Mandy to babysit, the baby bottle on the kid's table keeps appearing and disappearing between shots.
Continuity mistake: When Mandy is tied up and Nick comes to untie her, her pigtails is just covering the suspender on the right. When the angle changes it is covering the opposite one.
Continuity mistake: Whe Nick watches Mandy in the water park, the stuff on the trolley changes positions between shots in less than a second.
Continuity mistake: When Adam breaks Nick's guitar, the baby swaps sides in less than half a second between angles.
Continuity mistake: While Wayne is on top of the yellow Porsche's hood, the driver is looking up all the time. The immediate close-up shows him facing straight ahead and it's not until a couple seconds later when he faces upwards.
Continuity mistake: The yellow rope around Mandy's chest moves closer or further to the neck between shots.
Visible crew/equipment: Stage light is reflected on a glass door when the family chases the giant baby inside the house.
Continuity mistake: When Adam breaks Nick's guitar he leaves it next to the loudspeaker. In the next angle it's meters away.
Continuity mistake: After Wayne and Diane crash through the storage door, several metal parts get stuck on the van's roof. During all the escape, the parts keep falling and re-appearing back on the roof.
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.
Continuity mistake: When Nick and his dad are on the other side of the couch, hiding from giant Adam, the cushion keeps changing positions between angles.
Continuity mistake: When the (green-screened) giant baby breaks a part of the car and Nick picks it up, though it's similar, it's not exactly the same one.
Continuity mistake: When Mandy falls inside the Porsche, the Indian toy is facing an 8 o'clock direction, a second later it's facing a 6 o'clock one.
Continuity mistake: During the scene where Nick unties Mandy, the TV in front of the kid swaps from being normal size to really tiny.
Answer: She may have said goodbye to Adam but it just wasn't shown.