Continuity mistake: The movie supposedly takes place in April, however based on Chicago's climate, it is quite clear that the actual filming was conducted well into the summer months based on how full and leafy all the trees are, not to mention the ivy on the outfield wall at Wrigley Field which does not become fully developed with leaves until late May at the earliest.
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.