Harry gets Lloyd's tooth stuck in his head from slamming into him, but their faces never hit each other.
When Ms. Heller bribes the tour guide at the museum, the money alternates between being folded and unfolded.
Right before Ms. Heller sits down next to Principal Collins to watch the parade, the float can clearly be seen coming around the corner on the track. In the next scene, the float is back in the shed with Harry, Lloyd, and the rest of the class.
When Lloyd is sticking the napkins on himself in the cafeteria, he is wearing underwear under the napkins in some scene, but not in others.
Louis has a black eye after Lloyd hits him while in his pirate costume. Later that day, in the special needs class that same day, Louis doesn't have a black eye.
When Harry goes to Jessica's house at 7:00 pm, it is bright and sunny. Thanksgiving is coming up, and they're in New England, so it should be dark by then.
Jessica's father must have little to no sense of smell to confuse the chocolate for poop, as human excrement has a far more potent and pungent odor to the point where it's foul and offensive smell would have been detected well before opening the bathroom door. Not to mention, the sweet sugary scent of the cocoa used in the chocolate would have become immediately apparent with that much of it spread around the room.
When Harry and Lloyd run into each other, Lloyd visibly braces for impact when he's not supposed to know Harry is coming.
At several times during in the film, notably during the slushy scene outside the big A, you can see Lloyd's tooth through the blackout makeup.
When Harry and Lloyd are holding on the principal's car while riding in a shopping cart, the speedometer reads zero.
When Lloyd is driving the floor waxer on the sidewalk, wheels are visible on the bottom of people's shoes as they fall behind him.
On the first day of school, when Lloyd first points Jessica out to Harry, the footage of her runs in reverse for a second. When she turns her head, her hair moves before her head does.
Lloyd listens to Vanilla Ice's "Ice Ice Baby" on the school loudspeaker. The movie takes place in 1986; the song came out in 1990.
The convenience store has products in 2003-era packaging.
Ms. Heller bribes the museum tour guide with a post-2000 $5 bill, in 1986.
When Harry and Lloyd are outside the convenience store, current magazines are on the shelf. One of them shows Chelsea Clinton as an adult, although she was only 6 years old when the film is set.
Good Charlotte the anthem song played during the special recruitment scene but the movie takes place in 1986.
During the Thanksgiving Day Parade, the principal claims the band sounds terrible because he sold all the wind instruments, yet wind instruments are being played.
In the opening shot of the school carnival, the camera pulls back from in front of a goldfish bowl. The camera is reflected in the bowl.
When Harry enters Jessica's house, a mic can be seen through his shirt.
The convenience store awning advertises beer and wine. Convenience stores in Rhode Island can't sell alcohol.
When Principal Collins is driving Jessica, the skyscrapers in his rear-view mirror are in Atlanta, Georgia (most notably the cylindrical Peachtree Plaza Hotel), not Providence, RI.