The (onscreen or offscreen, depending on the adaptation) wedding scene. Petruchio shows up in what can only be described as "get-up", accompanied by Grumio, looking equally ridiculous. Undaunted by everyone's shock and embarrassment, he runs in and marries Kate in the most absurd manner possible, then proceeds to kidnap her before anyone can go to the reception.
After the Zany Scheme of the subplot finally blows up, everyone involved except Lucentio just books it rather than hang around to explain. It's even funnier when you have the script in hand:
Hortensio's monologue about getting hit with the lute.
During the wedding scene in the Zeffirelli version, as Petruchio and Kate make their wedding vows, Kate blurts out, "I WILL N–!", only to be Kissed Mid-Sentence by Petruchio. The priest, sensing trouble, hastily pronounces them man and wife as the kiss takes place.