The slap Ferris gives Cameron out of nowhere when he makes the situation worse.
"You have reached the Coughlin Brother's mortuary. We are DEEPLY sorry we are unable to answer your call, but if you LEEEAVE your name and number, we'll get back to you as soon as is humanly possible."
The entirety of the art gallery scene. Especially this◊.
Jeannie to the intruder (actually Principal Rooney): "Excuse me: if whoever was in this house is still in the house, I'd like you to know that I've just called the police. I'd also like to add that I've got my father's gun and ascorchingcase of herpes."
And before that, Jeannie and Rooney think Ferris is in the house, before Jeannie jumps out to catch him and Rooney jumps out and she screams and kicks him in the face three times. And then the cutaway scenes while Rooney collapses and Jeannie runs up to her room screaming her head off.
Talking about Cameron's car.
Ferris: Cameron, I'm sorry, but we can't pick up Sloane in your car. Mr. Rooney would never believe Mr. Peterson drives that piece of shit.
Cameron: It's not a piece of shit.
Ferris: It is a piece of shit. Don't worry about it, I don't even have a piece of shit; I have to envy yours.
When Rooney rings the Bueller's doorbell, particularly the second time Ferris' voice starts playing.
Ferris' recording: Who is it?
Rooney: You know damn well who it is!
The parking garage employees making the Ferrari fly over the camera as the Star Wars theme plays full blast.
The cut to Ferris about to play the clarinet: "Never had ONE lesson." *plays clarinet horribly*
Ferris disguising himself from afar as Sloane's father to get her out of school. Then tops it by making out with her in front of Rooney ("Do you have a kiss for Daddy?"), who is squicked out but walks away.
Rooney frantically running down the hallway, then slowing down to a speed-walk as he passes the classroom doors.
When everybody but Ferris gets home:
Tom: What's wrong? Katie: Everything. I just picked up Jeanie from the police station. (...) She got a speeding ticket, another speeding ticket, and I lost the Vermont deal because of her. Tom: I think we should shoot her.
When Ed Rooney is on the phone: "Grace! GRACE! GRAAAAAACE!!!!"
A truck honks the horn to "Shave and a haircut", and Rooney gives the finger to the beat of "two bits".