At the drug store the register changes from 10 cents to no sale, back to 10 cents and finally 15.
Also at the end of the scene the soda jerk has a fly crawling down his face, but when he hits it with a bottle the fly is gone.
When Joe enters the diner and is greeted by the waitress, he sits right at the corner of the bar in the close up shot. Seconds later, when the view switches to the long shot, he is on the third stool from the corner.
During the ice cream parlor scene, when W.C. Fields blows the foam off the ice cream soda, part of it lands on his hand. In the subsequent close up shot, when he picks the soda up, his hand is clean.
When the ladder of the fire truck lifts the car into the air, a shadow on the front of the building reveals the rigging and crane that actually did the lifting.