Continuity mistake: When Ferris turns off the shower, the steam disappears in the close-up angle, only to reappear half a second later.
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.
Other mistake: Ferris wants to skip school and who can blame him? In the boring lesson early in the movie, supposedly at the start of the day, look at the student's watches. They all read 6.25. (00:11:10)
Continuity mistake: When Ferris calls Cameron, the mess by his phone changes between angles.
Continuity mistake: In the scene where Ferris is talking on the phone with the freshmen and playing his "sick sounds" on the keyboard, his legs cross and uncross several times between shots.
Continuity mistake: When Rooney first arrives at the Bueller house, he parks in the street by the fire hydrant with a good deal of room between the car and the curb, however when the policeman is putting the ticket on his car, he is right up next to the curb.
Continuity mistake: When Jeanie and Charlie Sheen are talking on the couch in the police station, Jeanie's legs go from crossed left over right to right over left and her arm goes from resting on the couch to by her side and back again.
Continuity mistake: Sloan's hairstyle changes from parted to the side, to the left and straight back throughout the scene in which she says goodbye to Ferris.
Factual error: The odometer only goes up by 10 miles between the time Cameron freaks out and when they're back in the garage, but when Cameron freaks out they're at a famous Chicago lighthouse that is 1/2 hour outside the city, more than 10 miles, so the odometer should have increased by a larger distance.
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.
Continuity mistake: Cameron's shirt with the The American Medical Association's symbol changes half way through the movie. You can see how they try to cover it up by turning it inside out, but it still shows.
Continuity mistake: When Ferris talks to Cameron and plays with the mouse, the keyboard comes closer to the edge of the table between shots.
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: Ferris is messing with the equalizer on his stereo system (the one that plays the snoring), while he's talking to the camera. Watch the levels on the equalizer, they change from the close-up and back to normal in the wide shot.
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.
Continuity mistake: After Ferris plays with the synthesizer, the principal is fidgeting with some pages. The way they're folded changes between angles.
Continuity mistake: When the movie starts and Ferris' parents leave the room, his right arm swaps from being over the blanket to under, between frames.
Answer: Sloane knew it was a hoax. Ferris would not have done something like that without her knowing. Rooney pretty much had figured it out, but could never prove anything. Once he was discovered inside the Bueller house (illegally) by Ferris' sister, anything he did know he would have to keep to himself.
raywest ★
Sloane starts to put her jacket on the moment that she sees the school nurse walk into the classroom, suggesting that Sloane already knew she would be called out, and she smiles to her classmates.