Leo's Evil Laugh after he has Bob committed, especially since it's easy to see that his smugness is about to be crushed. He gives off an even more hilarious one after he straps a time bomb to Bob and runs off. Given the level of slapstick humor in the film (with Leo being the unfortunate target), you'd almost expect him to run into a tree branch or fall over or something.
The Angrish way that Leo tells Bob to get out of his car.
When Leo tells Bob to express himself during their first session, the latter lets loose a stream of curse words, with Leo's secretary listening with a bewildered look on her face.
On the day of his interview, Leo tries to wake Bob up and get him to leave, but his increasingly loud and physical attempts to awaken Bob fail miserably. Then the alarm clock goes off, and Bob shoots right up.
The mental hospital scene where Bob tells jokes to other patients and staff members was already funny, but it becomes downright hilarious when you realize that the entire scene was ad-libbed by Bill Murray.
In the same scene, when Dr. Tomsky tells Leo that they have no reason to keep Bob there: