When Annie and Rhodes are on the side of the road with the radar, Rhodes says for her to aim at the license plates of the cars, which gives the best reading. This is not true, as radar is a frequency that emits a cone that will respond to any moving object within range, and its range is infinite. Radar doesn't react to reflective license plates, but to moving objects.
When Annie receives the shower invitation it reads "Sat." as the day. When Annie's car breaks down on the day of the shower, and Ted picks her up, she asks him, "Were you busy?" and he answers, "Well, it is Friday."
When Annie is asking Rhodes to help them find Lillian, when Rhodes is standing at her car door he has what looks like a couple days worth of stubble on his face, but when he is back in his squad car, he is clean-shaven.
When Officer Rhodes is talking to Annie and Helen on the side of the road in the shot looking into the car (from Rhodes' POV) his hand is on the frame of the car. But from Annie's perspective (looking at Rhodes) his hands are by his side. This happens numerous times throughout the scene.
While Annie is making the carrot-shaped cake, she uses a liquid measuring cup instead of the dry measuring cups for the flour. She pulls them out of the kitchen drawer but the next shot she is using a liquid measuring cup to pour flour in bowl.
When Lillian is opening her gifts at the bridal shower, Megan's hand brace is on the wrong hand. During this scene it was on her right hand, while during the scenes before and after it was on the left hand.
When Annie and Helen are singing a duet of "That's What Friends Are For", during the last verse, Annie has the champagne in her left hand and the microphone in the right. In the next shot, she has them reversed.
When Ted pulls up to give Annie a ride after her accident, his car seats have the optional Porsche crest on the headrests. But in the interior shots after she gets in, the crests are gone revealing that two different cars were used.
After Annie leaves the bridal shower, she is seen driving down a two lane rural road. Megan, with numerous puppies in her van, pulls up next to and passes Annie. The street they are driving on suddenly has changed to a five lane highway with a more urban background. Megan pulls in front of Annie and speeds away. Seconds later, Annie slams on her brakes to avoid hitting a porcupine, and once again is on a two lane rural road.
The plane seen taking off to Las Vegas has two engines and appears to be a Boeing 767. However, when the plane makes an emergency landing in Wyoming, the plane seen landing is an L-1011 (three engines; the third mounted via S-duct in the tail).
The overhead shot of Annie driving to Chicago for the engagement party, shows signs for Madison, Chicago and Green Bay. The car she's driving is headed toward Green Bay, which is north of Milwaukee, so she's in the wrong lane. Chicago is south of Milwaukee.
Twice throughout the film the Officer Rhodes says the names of intersecting streets in which characters live in Milwaukee, but when the location is shown, the street signs have different names on them.
When Annie and Rhodes are on the side of the road with the radar, the radar is actually a Kustom Signals HR-12 model which is not wireless. The cord has been removed, meaning that what Annie is holding is a dead, non-working unit.
(possibly intended) At the bridal shower, the waiter offers Annie a glass of champagne, saying 'It's French'. All champagne is French, otherwise it is called 'sparkling wine'. It is possible, though, that Helen's snobbery and wishes to impress had her tell the waiters to specify to the guests that the champagne did indeed come from France.
The Wilson Phillips CD Lillian gets as a present is a greatest hits compilation featuring the song "Hold On", rather than the original album this song appears on. It isn't the wrong CD, just a different one.