When Fred and Ethel come upstairs to 'throw in the towel' in the lease battle; Fred has a light fixture on his head and shoulders that apparently fell on him. It was sitting in an upside down orientation; 180 degrees from what it should have been if it had actually fallen from the ceiling.
Throughout the series, Lucy's (allegedly) atrocious singing voice is a recurring plot point, but when she's singing with Ricky and the Mertzes (as well as in episode California, Here We Come! (1955)) her singing voice sounds fine.
When Fred and Ethel leave the Ricardo apartment after singing Sweet Sue, they go off in separate directions in the "hallway."
When the band is playing in the Ricardos' apartment, a flash can be seen from a flash photograph taken by an audience member.
When Lucy suggests that Ricky have his late-night rehearsal in the apartment instead of the club in order to annoy the Mertzes into breaking their lease, Ricky at first scoffs: "Honey, are you crazy? I've got a sixteen-piece band. I'll blow the roof off the joint." When Lucy counters that "It doesn't look like rain", he decides to take her suggestion. The band in the apartment, however, is missing a few from Ricky's 16 - there are thirteen - four saxes (doubling flute), 3 trumpets, 2 trombones, 2 percussion, bass and piano - fourteen if you count the harp, which is there, but nobody plays it.
Lucy says, "Gee, Ethel, you certainly play the piano wonderfully!" But elsewhere in the series, such as Lucy's Club Dance (1954), Ethel is shown to be an atrocious piano player, so bad that Fred wears earmuffs to drown out the sound.