When Jack's mother comes to visit Mary, you can see in the background what appears to be a little boy standing in a doorway. There is a rumor that this is the ghost of a little boy who died in the apartment in which the film was shot. This rumour is false, as the interiors were all shot on a sound stage in a movie studio. The "ghost" is actually a cardboard cut-out of Jack wearing a tuxedo. This prop appears later in the film, when Mary's mother comes to collect her.
Toward the end of filming, the babies that play Mary, were less entranced by the actors around her, and became enticed by the microphone. She would start to follow it with her gaze, and not look at the actors. Leonard Nimoy and other crew members had to start hiding the microphone and disguising it from the baby's view.
Director Leonard Nimoy looked at videotape footage of about 200 hundred sets of twin girls for the part of Mary, before deciding on Lisa and Michelle Blair for the part. Four sets of twins were seen in person before the final choice was made.
The "greatest clutch shot in basketball history" that Peter's guests are watching during the party is from Game 3 of the 1970 NBA Finals. Jerry West of the Los Angeles Lakers sank a 65-foot desperation heave with two seconds left to tie the game, but the New York Knicks won the game in overtime and eventually the series to win the NBA championship. Largely unknown is the fact that, after the Knicks went up by two before West's shot, 'Wilt Chamberlain' never was completely out of bounds after he took the ball to inbound it to West. Chamberlain was frustrated and just casually flipped the ball to West and was heading toward the locker room when West hit the shot.
The party scene at the beginning of the movie was going to start differently. One of the extras was going to be on the phone in the lobby, hang up, and then be followed into the party. The extra was unable to get the shot right, and so that entry was scrapped.
In the original screenplay, Jack was an airline pilot. Paul Millner had a larger role. The overall film was originally intended for adults as there was much more cursing, a big animosity between Peter and Michael. Also when Peter returns with the formula, originally Michael then went out to Safeway to get the diapers, which Peter forgot.