Revealing: In Season 5 - Burnt Out: when Arnold's burns down you can see the set walls wobble a lot.

Continuity: In episode 136 (Marion, Fairy Godmother), Ralph is at Al's after everyone else has left on their dates. He's depressed and as he walks up to a table, you can see a couple of plates of food sitting on it. Then Fonzie comes in (hitting the wall to turn the lights on) and Ralph looks away from the table (he doesn't move otherwise). Then they show the table again - which is completely empty.

Continuity: In Arnold's, throughout the series you can see the Men's toilets on the left and the Women's toilets on the right. But whenever Richie, Fonzie or any boy goes into the Men's toilets, they make a right.