In Act 3, after Lucy is soused on the alcohol in the tonic, Ricky begins to sing a song on the "live" show. Lucy enters from the left, between scenery flats, and in the darkness behind her, you can see the Ricardo's apartment fireplace and mantel.
At the start of Act 3, Lucy can be seen twice waiting off stage for her scene to start.
You can hear Desi Arnaz's distinctive laugh offstage along with the audience as Lucy becomes more and more intoxicated.
When Lucy begins to rehearse her commercial, on the Vitameatavegemin sign behind her, the tape forming the black border is coming apart on the upper right corner.
When Ricky begins to play the guitar, between two walls of the set behind him, you can see the Ricardo's living room mantle/fireplace and brick wall, and you can see the silhouette of Lucille Ball waiting to make her intoxicated entrance.
While Lucy does her "TV commercial" you can see Desi is trying very hard not to break character and laugh.