Barney Mends a Broken Heart - S3-E6
Visible crew/equipment: While Andy and Barney are with Daphne and Skippy at the Tip-Top Cafe in Mount Pilot, when Daphne's boyfriend walks in we can see the actor's chalk T-mark on the floor.
The Andy Griffith Show (1960)
1 visible crew/equipment mistake in Barney Mends a Broken Heart - chronological order
Starring: Ron Howard, Don Knotts, Andy Griffith, Frances Bavier
Visible crew/equipment: When Otis tries to escape from Aunt Bee's clutches by hiding in the laundry service van, just as Aunt Bee has the driver open the van's back doors we can see the reflection of the moving boom mic in the left window.
Opie's Group - S8-E9
Andy: Clara, sometimes a parent can't see what he should do, and sometimes it takes a person from the outside to show him. And I'd like to thank you.
Clara: Groovy.
Trivia: When Gomer and Barney are waiting at the Miracle Salve Co. Office, Barney speaks in a nasally high pitch voice. When Gomer asks Barney what's the matter with his voice, Barney explains that he's been talking like that as a disguise, to which Gomer asks, "How do you do that? I wish I could do my voice like that!" Jim Nabors does precisely that for his role as Gomer.
Question: Why is it when they were on the show the Darling sons never spoke? Would they be paid more if they did, or was this some sort of joke?
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.