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).
Suggested correction: While not likely, there is no requirement that the phones have the same lines or be in the same order, so the line assigned to the third button on Grace's phone could be the second line on Rooney's phone.
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.
Suggested correction: They took a detour to think at the lighthouse disregarding the distance, since at this point 30 extra miles isn't going to show against the damage already done.
