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: The odometer only goes up by 10 miles between the time Cameron freaks out and when they're back in the garage, but they stop at a famous Chicago lighthouse when Cameron freaks out that actually is 1/2 hour outside the city.