Cosmo running up and back-flipping off the walls at the end of "Make 'Em Laugh".
What happens at the end, when Lina's voice is revealed as a fraud.
The sheer glee on Don, Cosmo and RF's faces as they mime along with Lina's idiotic dance moves, right before they raise the curtain to reveal Kathy singing behind Lina.
Near the end of the "Broadway Melody" sequence, we see Don's character sulking after being rejected by his crush, again. Suddenly we hear a voice sing the exact same thing he did at the beginning. Then the two look at each other for a moment, the hoofer just shrugs and walks off. But Don's character has an epiphany and sings.
The sudden and unexpected appearance of Cyd Charisse's legs in that sequence and the dance number they lead to. There's a reason that shot is used as the page picture for She's Got Legs.