Continuity mistake: When Cameron is on the phone with Mr. Rooney pretending to be George Peterson, Ferris calls the school. Grace picks up the phone in the outer office and tells Ferris to hold. You clearly see the third button (line three) is the one he called on. Grace says that Ferris is on line two. When Mr. Rooney puts Cameron on hold and listens to Ferris, after Ferris hangs up, the camera cuts to the phone buttons on Mr. Rooney's phone showing that Ferris was just on the second button (line two).
Factual error: Cameron tries to stop Ferris from borrowing his father's red sports car, explaining that it's an extremely rare and valuable '61 Ferrari 250 GT California. But the car itself is not a real Ferrari. It was manufactured by Modena Design and Development of El Cajon, California, and is a "Modena GT250" which strongly resembles the 1961 Ferrari 250 GT California Spyder. Since the film makers were throwing the car out of a window, using a real Ferrari was 'cost-prohibitive'. Comparing the real Ferrari to the Modena, differences include the Modena's sharper body crease above the grille, the more steeply angled headlamp covers, higher-mounted emblem and counter-sunk hood scoop.
Continuity mistake: In the scene where Ferris is singing "Twist and Shout," you can see a building sign that reads '33 North Dearborn Street.' After the shot cuts, you can see an intersection of two streets: Adams Street and Dearborn Street. The address '33 North Dearborn Street' is a few blocks away from the intersection.
Continuity mistake: When Rooney gets on the school bus, he crosses a grass boulevard with no sidewalk to board it. The bus never moves. When the bus pulls away, there is a sidewalk next to the bus where the grass was.
Continuity mistake: In the scene where Ferris catches the foul ball, the television shows the stands where Ferris & company are sitting (down the left field line) as being in the shade. When the television shows the close-up of Ferris catching the ball, he is in the sun. When they have the actual shot in Wrigley Field, it is overcast.
Visible crew/equipment: After Ferris picks up Sloane from the school and they leave the parking lot, there is a shot from behind the car as they drive away and Cameron emerges from his hiding place. The following vehicle and the camera boom are visible in the reflection on the car's trunk. (00:31:15)
Continuity mistake: Whilst Jeanie and the guy in the police station are talking, a man in a suit walks behind them and then takes a file from the woman at the desk. However, without walking behind either of them, he ends up on the other side of the room.
Continuity mistake: When Jeanie says 'I don't know why I'm here', there's no one behind her. However, in the next shot of her, about a second an a half later, there are three people behind her.