When I saw this episode as a kid, I was mostly fixated on Potsie's poor singing and the weird new Arnold's set (just before Ron Howard left, Garry Marshall ordered Arnold's burned down so the set could be rebuilt in a style that '80s kids could relate to). Now I'm fixated on something else: the uncredited guy in the blue sweater, whose dancing is prominently featured in this clip. It may be the worst dancing I've ever seen on a TV show, except for dancing that is intentionally supposed to be terrible. I guess it's possible that Jerry Paris told him to dance that way to add some comedy to the scene, but it doesn't really come off; it's just bad spastic hair-coiffing dancing.
But he dances so badly with so much enthusiasm that he's almost a predecessor of Jeff from Mr. T's "Be Somebody Or Be Somebody's Fool." Never have I found it more unfair that a show could get away with not crediting someone if it didn't give him any lines. This man deserves to have his name, and his shame, preserved for posterity.