The Walker's Drug Store set has a pay phone mounted on the wall. The pay phone has a dial. Mayberry doesn't have dial telephone service. The other telephones seen on various sets have no dials, either, only this one.
When Mr Mason expresses interest in buying the old copper tub on the counter, Ellie agrees, saying "I don't have much use for it!" But it's sitting on the counter, being used as a display to hold bars of soap. In fact, Ellie has to empty the tub so Mr. Mason can take it away.
Opie plans to trade "licorice seeds" for a pair of roller skates, but he should already have skates, as Ben Weaver had gifted him a pair on Christmas Eve in Christmas Story (1960), just three episodes ago.
Andy tries to get Barney to buy the cannon for his front yard. However, Barney rents a room in Mrs Mendelbright's boarding house. He does not have his own house.
There's no government body in Mayberry that would explain having both Barney and Andy as voting members.