The song the "guest vocalist" sings at the high school dance has multiple instances where 2 or 3 individuals can be heard singing together but none of the band members are depicted as providing the required backing vocals.
The position of the Greg's guitar strap keeps alternating between being under and over his collar when he's playing for Cindy. The same thing happens moments later when he's playing while watching himself in the mirror.
In the beginning of the movie, when Cindy goes to the Dittmeyers to swap the mail, there are no vehicles in the driveway, nor in the garage. However, when Mike leaves for work, his car is in the driveway.
Mike drags Carol's needlepoint to his side of the bed (the right side), but when Cindy interrupts what Mike and Carol are doing, the needlepoint is on the left side of the bed where Carol originally was.
When Marsha and Doug Simpson are on the hill in the car and he tries to make out with her and she tells him she is not that type of girl, her nose is back to normal, although it was clearly squashed before that scene and all the scenes following.
When Cindy Brady delivers the Dittmeyer's mail, Larry Dittmeyer is running on his treadmill. The Bloody Mary, in the drink holder, has no celery stick in the glass but, in a following close-up, a celery stick appears in the drink from nowhere.
When Greg and Marcia are accosted by the "carjack" criminal, Greg rolls the driver's side window to his eyebrow level and talks through the glass. Seen from inside the car in reverse angle, he has rolled it down to below his chin.
When Jan runs into Mr. Ditmeyer's car and breaks his mirror, she is dressed in the same outfit (green turtleneck, crochet vest, and blue plaid pants) that she wore in the opening scenes of the movie, but all the other characters have already been seen dressed in different outfits, suggesting it's a new day.
When Marcia goes to her drawer to see if Jan had taken her knee socks, you see her open the drawer with the left hand. However, when a close shot is shown it appears she has opened the drawer with her right hand.
Mr. Dittmeyer is looking through the driver's side rear view mirror while talking on the car phone. The mirror shows that "objects are closer than they appear" just before Jan knocks off the mirror riding her bicycle. This warning is only on the passenger mirror.
While walking on his treadmill, Mr Ditmeyer's tomato juice glass is being jarred quite a bit, yet does not fall. After getting off the treadmill to get the Brady's mail, the mere movement of the still-running treadmill belt is enough to make the glass slide off, spilling the juice on the belt indicating the glass was anchored to the console initially.