The cosby show was always re-using actors. Martin's father was played by that same guy from the episode when Justine kept seeing other guys and Theo wanted to use some voodoo to get rid of the guy. They went to some voodoo man and he kept saying "27 years, same location. I'm not going anywhere, and no one's asked me to." Later on, he played Martin's dad.
Also, a friend of Cliff's and Russel's had come over for Russel and Ana's anniversary. He was the same guy who played the waiter in an earlier episode, when Cliff takes Rudy, Kenny, and some of their friends to a gourmet restaurant and they wind up eating as Circus Burger.
And who can forget Jade? Denny's over-exaggerating girlfriend who magically returned with a Barbados accent as a completly different person, and wound up being Theo's girlfriend? Not only that, there was a scene when the girl and Denny are in the same room, and it's like they've never met.
Can anyone spot anymore?