The car the Draytons are driving is a two-door sedan (with a pillar holding the roof up, and a full frame around the door glass) when seen in exterior shots. Once they're at the drive-in, it becomes a two-door hardtop (no pillar, and no frame around the glass).
When Matt and Christina are at Mel's Drive-in, the rear view projection of the street shows it running perpendicular to the parking lot. During the actual shots of the property, the street runs parallel to it.
The car that Tracy backs into already has a dent in the front door panel. During the accident the relative position of the two cars and the number of dents in the side panels change with each change in shot.
When Mrs. Drayton, her daughter and John Prentiss first sit down in the terrace together, Mrs. Drayton's fingers are inter-laced and she's touching her chin. A moment later, her hands are folded and across the plate that's in front of her.
When Matt is dressing before dinner and talking to Monsignor Ryan, he begins to tie his tie. When the camera switches to a front view of him, he hasn't begun to tie it yet and is still buttoning his top button.
The aircraft shown in the opening sequence is a United Airlines DC-8 and is supposed to be the same flight. However, the aircraft shown flying over San Francisco has registration number N8022U and the aircraft shown landing has registration number N8035U.
One steak short for dinner (Monsignor Ryan). When Joey finds out Monsignor Ryan is coming to dinner she goes to the kitchen and sees Tillie making a pie and states, "I hope you made an extra pie because Monsignor Ryan is coming to dinner too." At no time did Joey tell Tillie to order another steak for Monsignor Ryan like she did when she found out John's parents were coming to dinner and had Tillie order steaks for them.
When Joey turns on the kinetic sculpture, the switch seems to be on the right side (as viewed from the gallery entrance), but when John turns it off at the end of the scene, he reaches to the left side (and perhaps a bit to the back).
Dr. Prentice is in the study without Joey explaining to Mr. and Mrs Drayton behind Joey's back that he will not marry her without their blessing. The books on the right side of the shelf behind the Draytons (especially the one with the red spine on the end) are being held up straight by the small bronze bust. After Dr. Prentice leaves the bust is pulled back to be more centered on the shelf between the books and now the books to the right of it are being held up straight by leaning the book with the red spine against them.
When Mrs. Drayton is in the garden with Mr. Drayton picking yellow roses she puts one in his lapel and the stem is poking out from behind his lapel. After she says "Guess who's coming to dinner?" they show a close-up reaction shot of Mr. Drayton and the stem is hidden. Then the following shot shows the stem again before he pulls the rose out in anger.