Raoul confesses something he learned while eavesdropping on Christine for the second time. To paraphrase, a very annoyed Christine asks, "You were listening at my door again?", and (to further paraphrase) Raoul answers, "Of course not -- I was hiding in your closet."
Richard's and Moncharmin's attempts to catch the Phantom the night he extorts his second payment, which includes walking backwards all night and the crazy incident with the safety pin that makes the poor secretary Remy think his bosses have lost their minds.
Erik's sarcastic letters to the managers near the beginning, particularly the names he calls the other performers.
Your Mileage May Vary but my sister and I were giggling hysterically during "Stranger than you Dreamt" as the Phantom crawled across the stage to Christine-- The Phantom was such a drama queen.
Piangi attempting to scale the mechanical elephant in the "Hannibal" scene.
Carlotta's diva rant shortly thereafter ("'These things do happen'? You've been here five minutes, what do you know?!").
The very long low note done by the old cuckold in "Il Muto".
The low note has, by now, actually earned the cuckold his own round of applause in most theatres.
"The role of the pageboy is silent, which makes my casting, in a word... ideal."
Michael Crawford's delivery of that line on the London cast album is delightful. And just try not to smile a few minutes later when he spoils Carlotta's performance and accentuates it with a wonderful Evil Laugh.
This little bit in the song "Notes/Prima Donna". "And what is it that I'm meant to have wrote?" Beat "Written."
It doesn't come across very well on the original cast recording (or the movie, unfortunately), but the managers can be absolutely sidesplitting in the right hands. Firmin Deadpan Snarking his lines in "Prima Donna" or Andre fumbling his way through the "ballet from Act Three" speech are among the many delights Those Two Guys bring to the show.
I happen to find nearly everything Carlotta says or does to be so over-the-top that she is one of my favorite Characters That Don't Matter Much in The End. Especially her aforementioned rant after Hannibal, especially how it was done in the film: "For the past three years these things do happen? And do you stop them from happening? No! And you two! you are as bad as him! 'These things do happen?' Ma, hmph! Until you stop these things from happening THIS THING-" *points at self dramatically* "-DOES NOT HAPPEN! UBALDO! ANDIAMO! BRING MY DOGGY AND MY BOXY. Now you see, bye bye, I'm leaving!"
Kind of darkly funny, but this exchange during the "Don Juan Triumphant" rehearsal never fails to amuse me:
Carlotta:(re: Piangi's incorrect singing of the score) His way is better; at least he makes it sound like music!
Mme. Giry: Signora, would you speak that way in the presence of the composer?
Carlotta: The composer is not here! And if he were--
Mme. Giry: Are you certain of that, Signora?
cue massive Oh Crap take into the flies from everyone onstage*
The 25th Anniversary's version of this scene has a couple of gems. First is Piangi's line: "If you can call this sh--gibberish art!" And, second of all, La Carlotta parades around the scene wearing an enormous Jammie Dodger on her head.
From the Arthur Kopit/Maury Yeston musical, we have the Phantom's initial reaction on hearing Carlotta (in this version a Dreadful Musician who only gets leads because her husband is the manager):