Revealing mistake: When Mandy falls inside the Porsche, her stunt double is noticeable because she is way older than her.
Continuity mistake: Whe Nick watches Mandy in the water park, the stuff on the trolley changes positions between shots in less than a second.
Visible crew/equipment: Stage light is reflected on a glass door when the family chases the giant baby inside the house.
Continuity mistake: In Las Vegas, when the baby leaves the car its position keeps changing between shots.
Revealing mistake: When the family is running around trying to settle down the big Adam, Adam picks up Nick. From the back view, Nick is obviously a doll.
Revealing mistake: When Wayne and Nick stare at the giant baby entering the room (a superimposed projection), they keep staring ahead for a couple of seconds despite the baby already having turned right. The cue to where to watch failed.
Revealing mistake: After the mother sees giant Adam and faints, Wayne is replaced with a doll with chubby rubber arms.
Continuity mistake: In Las Vegas, when the ice-cream truck is about to play the music, the position of the baby's feet, the lettering on the billboards and the people next to him change completely between shots.
Revealing mistake: When Mandy is hanging loose by the car's edge and Nick is grabbing her, there's a shot with a bluescreened background where Nick says "but you gotta help" where Mandy and the car are all blurry and faded and the image is doubled. Something went really wrong in that special effects shot.
Answer: She may have said goodbye to Adam but it just wasn't shown.