A Quiet Day in Dodge - S18-E19
Other mistake: Festus returns from catfish fishing carrying a gunny sack full of fish that is bone dry when it should be dripping wet or at the minimum damp from the fish.
TV or Not TV - S1-E1
Other mistake: At the beginning when Ed tells Ralph he can't afford to buy a new TV right now, he says it couldn't come at a worse time because Captain Video is blasting off to Pluto tomorrow. Ralph gets the idea to buy a set together. When Ed comes down and turns on Captain Video for the launch to Pluto, Ralph yells at him saying they've had this set for three nights and all they watch are Ed's shows. Captain Video should have blasted off for Pluto night one, not night three.