Continuity mistake: When Rooney is at the Bueller house, he slips off the water spout into mud. His shoe is stuck in the mud when he pulls his foot out. Yet, there is no mud on his calf, which was also in the mud.
Continuity mistake: When the flower deliveryman goes to leave the Bueller house, he toots the horn before speeding off after Rooney flips him the bird. It then cuts to a wide shot of the front of the house and the van is gone. We never hear the van's tires clearing the bump between the driveway and the road, and the van would've had to be going excessively fast to have cleared the driveway in the time between hearing it set off and the wide shot.
Continuity mistake: If you look closely every time a clock is shown you can tell they do not match up. The times match up at the beginning but then proceed to jump back and forth throughout the movie.
Continuity mistake: The dash or speedometer of the Ferrari is visible multiple times, but two different dashes are seen. In one the speedometer has a top speed of 160 (garage men speeding down the road, when Cameron sees the mileage is too high, and when Cameron sees the miles aren't coming off). In the other it has a top speed of 120 (when they get in the car to leave the garage, and looking over Ferris' shoulder at Cameron leaning on the car in the garage before he kicks it [open matte version]). The two dashes also use a different font for the numbers. (00:55:00 - 01:09:40)
Continuity mistake: After Ferris picks up Sloan from school in the Ferrari, he is seen speeding off towards the end of the parking lot (there's an exit and then a few more stalls before the end of the lot). He gets into 2nd gear before it cuts back to a shot of Mr. Rooney. As we see Rooney's face, we hear the Ferrari running through 2nd gear and into 3rd. This is impossible as they would've had to slow down to make the exit or they would've run off the end of the lot.