The braces that Maureen McCormick wore were fake. In real life, she didn't have to get braces. Eve Plumb, Susan Olsen and Mike Lookinland all had to get braces in real life.
When Alice returns Marcia's dress to the dress shop, the master bedroom set was used as the dress shop. Curtains were hung to hide Carol's dressing table area and a room divider is used to disguise the closet doors, but the room is identifiable as the master bedroom by the colors and bricks.
Maureen McCormick stated in her memoir, "Here's The Story", that the tears were real when Marcia was crying in the mirror, as she was going through personal issues in real life.
The dress that Maureen McCormick wears to the dance shows up in "Here's Lucy": season 5, episode 11 ("Lucy And Donny Osmond"). Eve Pumb wears the dress as Lucy's niece, who meets Donny.
Florence Henderson's role in this episode is significantly reduced, as she was filming "Song of Norway" at the same time.