While Dave the Dude's gang waits inside Missouri Martin's nightclub, Happy McGuire and Dave stand outside and are informed of the presence of the cops. Behind them on the left side of the double doors there is a "Closed" sign but the sign is gone when the interior shot has the two entering the club.
The position of the pool/billiard balls changes between shots in both the pool hall scene and the billiard room scene (obviously to set up the trick shots that follow).
Half the contents of Apple Annie's liquor bottle disappears between gulps while she reads the letter she wrote to her daughter.