At the end of Lucy and Van Johnson's dance, Ricky and the Mertzes are in the audience applauding as Lucy runs to head backstage. In the next shot, as Lucy runs through the curtain, you can catch a glimpse of Desi Arnaz standing behind it waiting for his cue.
Lucy flubs her line when she tells Ricky that she danced with Van Johnson. She starts to say that she "Vanced" instead of "Danced".
Fred is supposed to say "I guess you can dance just as good". He instead says "Jance" instead of dance.