The ages of the babies in the final scene are inconsistent with the time frame and the birth orders.
The horse seen at the beginning of Kevin's birthday party has a small sliver of white on its forehead, the horse that Gil rides off on at the end of the party has white all the way down the front of its head.
In the classroom scene where Nathan sings to Susan, she writes BONUS across the top of the blackboard. It is written much smaller in some subsequent shots, then goes back to the larger writing.
During the car ride back from the baseball stadium at the opening of the film, Karen is not wearing her seat belt. In the next shot she is wearing it.
When Helen gets out of the car after her carnival date with Mr. Bowman, she's wearing tan shoes. When she enters her house a minute later, she's wearing white shoes.
Julie is described as having an SAT score of 1291. SAT scores are multiples of 10.
In the first Little League game, the player scoring the winning run (2nd across the plate) does not touch home.
At 0:31:14 -the two beers opened don't foam. They are fake.
During one of the baseball scenes, a ball is hit into the outfield. The player appears to have caught it, but a ball appears on the ground beside him. Yet when he reaches into his glove to throw the ball home, it is there, while the ball on the ground is still in sight.
The dog at Kevin's birthday party (seen eating Justin's burger) is a Bassett hound. The dog in the living room the day Gil quits his job is some other breed.
The shoe shown in the vandalism scene is a solid-white shoe. Garry is always seen wearing black Converse high-tops. (And, as a vegan, Joaquin Phoenix still personally favors Converse high-tops.)
At drag race, you see Garry's hat fly off, as he runs toward Todd, when he crashes. But, when he arrives, you see Garry's hat back on his head.
When Kevin catches the ball near the end, the same audio of both boys falling and groaning was edited in twice.
Entire crew visible, reflected from car as Frank talks to Larry.
Steve Martin's stand-in is visible when "Cowboy" Gil rides the horse. The stuntman's wig is a different shade of gray than Steve Martin's own hair.
Reflection of boom mic can be seen moving on the trunk lid of Frank's classic car as he checks it for scratches after Larry has brought it back after taking it out without his permission.
The St. Louis Cardinals game they were attending was a spring training game. The Cardinals hold their spring training in Florida, so there is no geographical continuity error involving the Cardinals game.
Set in St. Louis, Missouri, but the license plates outside the baseball stadium during the opening credits are from Florida.
Another sign that the movie was shot in Florida rather than Missouri where it's meant to be set: the booth where Julie and Tod pick up the pictures is in front of a Publix, a grocery store found ONLY in Florida until 1991. The first Publix outside Florida opened in Savannah, Georgia, in 1991.
More proof that the film was shot in Florida, not Missouri: we see black lane markers on the freeway; the type of grass on the lawn.
A billboard for the Orlando Sentinel (local newspaper for Orlando, Florida) is visible at the baseball stadium, but everyone around Gil and his family are wearing St. Louis Cardinals clothes, foam fingers and inflatable bats. St. Louis, Missouri, is 1000 miles away from Orlando, Florida and Gil drives home from stadium, it's unlikely they (and everyone else) travelled all the way down to Orlando with young children just for a baseball game. Ergo the scene is presumably supposed to be set in St. Louis.
At 0:30:30 -a true automobile aficionado would not be grinding his metal watch band against the paint of an antique car.