In this episode Ethel is said not know the song "Sweet Sue" and she can't play the piano. In episode "Breaking The Lease" (S1E18) not only is Ethel playing the piano but she is playing "Sweet Sue".
For the calypso number at the end of this episode Ricky says that Little Ricky can play the bongos (attached single-headed, open-ended drums), Fred can play the quijada ("built with the jawbone of a donkey, horse or cow, and is weathered until the molars rattle in place"), Ethel the cencerro (cowbell), and Lucy the cuíca (a friction drum).
In "The Operetta" (1952) Ethel plays the piano while Ricky sings. They are rehearsing Lucy's musical called The Pleasant Peasant. Ethel does know how to play the piano as presented in that story line.
Lucy says she only knows how to play the song Sweet Sue on the Saxophone. In the Season 2 episode "The Saxophone", Lucy said she only knows how to play Glow Worm.
In this episode, Fred the dog is full-grown. In "Lucy Misses the Mertzs," which aired a month earlier, Fred is a 3-month-old puppy. There's no way that he could've matured that much in one month.