After Dwayne breaks the second half of Fonzie's table there is a reaction shot of Ashley and Roger. The bar next to Ashley is full of broken plates that Dwayne breaks later on. This shows that the reaction shot was taped after the fight sequence, a common technique in television.
When Fonzie goes crashing through the window at Arnold's we see a concrete block wall very close to the window. Arnold's is a stand alone building, meaning no other buildings are nearby, but the concrete wall was put in anyway in order to hide the back of the set.