Barney knocks the bell off the door when he sneaks out of Walker's Drug Store, but when Andy comes in a few minutes later, it Is back on the door and rings when Andy opens it. At the end of the episode, when Andy and Barney enter the drug store and Opie is in the back with Ellie and Dr. Benson, the bell is not on the door.
When Andy and Barney are talking at the soda fountain, the strap on Barney's hat changes position several times throughout the scene.
Early on, Dr. Benson states he either orders prescriptions by going to the pharmacy in person or by phoning them in, but he doesn't yet have a phone. No mention is made of simply writing a prescription, the general practice in the 1960s. He then proceeds to do just that for Barney after his examination.
When Andy is pouring a soda for Barney, the soda doesn't bubble up. That must be the flattest pop ever.