The Barbershop Quartet - S7-E3
Other mistake: The end credits list actor Burt Mustin as playing a character named "Jud", but while Andy and the rest are waiting for Howard to show up for practice, Andy, Wally, and Aunt Bee all use Burt Mustin's real name "Burt" instead of his character's name, "Jud." Then during the rest of the episode Andy still calls him "Burt" a few more times. (00:02:00)
The Barbershop Quartet - S7-E3
Other mistake: At the end of the show Andy talks about the upcoming bowling league. Howard shows Andy his injured left thumb. This should not affect his bowling since Howard is right handed.
Answer: It was a joke. They were supposed to be dumb hillbillies.
Both. According to later interviews with both Andy and the Dillards band (the real name of the band that played the parts of the Darling children, of which Charlene (Maggie Peterson - Mancuso was also a member) the reason the Darling boys never had any lines was because they would've had to have been paid if they spoke. In return for appearing without lines or pay, Andy promised to get as much of their music onto the show as possible which, he did. This obviously gave them a lot of exposure.