The part where Michelle must pretend to be a couple's counsilor. What makes this comical is that the roles are switched: Andie has to play the overemotional girl while her friend plays the sane one. It's In-Universe Playing Against Type.
After learning the truth, Ben and Andie have an argument in the form of a duet singing "You're so vain". Rather badly.
During their visit to his family in Staton Island, Ben's mother reveals that Andie is the first girl he's ever brought home to his family.
Even before that, Andie and Ben bonding over his family's favorite card game. It's the first time Andie drops her façade as a clingy, sensitive cringe-worthy girlfriend.
Ben's quiet delight at learning second-hand that Andie loves him. Not because he's won a bet, but because he loves her back.
Ben: She loves me?
When Michelle's boyfriend comes back to her, especially because, despite her clingy nature, he missed her during their breakup.
Designated Hero: We're supposed to root for both the leads even though they have no problem doing something so nasty to another person that could damage their self-esteem and ability to trust.
Fridge Brilliance: Why does Andie's boss take Andie's revised article about "How to lose a Guy in 10 days" anyway? Because it's close to what she wanted in the first place, a humiliating story about how a girl (initially Michelle) lost her boyfriend.
When first visiting his family in Staton Island, why does Ben initially fib that his family's swearing is Tourette's Syndrome instead of explaining it's a game? Because he was under the false pretense that Andie was crazy, and he didn't want to share his family's game with her: it's too special. (Also, this was the same girl who ruined Poker Night for him and his friends).
Even before Michelle's boyfriend comes back, there's one hint that Michelle's 'clingy' ways aren't truly sure-fire ways to lose a guy in 10 days. Remember when she was pretending to be a couples therapist and suggested Ben and Andie meet his family? That was her suggestion. And guess what? It only drove Ben and Andie closer together!
Jerkass Has a Point : Andie's boss at the magazine. It's a magazine devoted to fashion, makeup, and how-to articles. Why would Andie think "serious journalism" has a place there? Didn't she read the magazine before she took the job?
Tear Jerker: It's rather painful to watch as Andie and Ben simultaneously learn about the bet and the magazine article (respectively). They both fell in love with each other, and it hurts that just as they learned the other genuinely loved them, it's undone by the bitter truth.
Before this, Andie has a change of heart when she realizes she's fallen for Ben. He's unaware that he is part of her project to 'lost a guy in 10 days', and she's feeling awfully guilty about manipulating the one guy she's fallen for.
Why Would Anyone Take Him Back?: A double example in which both partners start the relationship under false pretenses. Both halves of the deception are revealed; both parties are hypocritically furious and dump each other, then decide in the end that they're meant to be together after all. Though it could be argued that in that case, they deserve each other.