A Black Day for Mayberry - S4-E7
Factual error: An armored truck is going to Fort Knox from Denver would never go through Mayberry. Fort Knox is in Western KY south of Louisville on I-65. NC is accessible mainly by going south down I-75 to Knoxville, then heading South East along I-40 towards Asheville, NC. To get to NC from Fort Knox you would need to go up to Lexington then Knoxville/or go east across the wky fka The Green River pkwy to I-75 then to Asheville. There is no easy route; why would they go through KY to NC and turn back to KY?
My Fair Ernest T. Bass - S4-E17
Character mistake: During the party at Mrs. Wiley's, Ernest Bass' girlfriend is called "Ramona." At the end of the show, Ernest T keeps calling her "Romina".
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.