That's true! I didn't realize that we got Caroll's cameo in episode 596 and then Frank's cameo in episode 581 all within the past month. Seeing Caroll's appearance was pretty funny, but it would be absolutely GOLDEN to see Frank's or the combined one with Richard and Jerry.
Does anyone know if Jim Henson ever made a cameo appearance in a street scene? I know he was in the "Song of Three" film as a juggler, but I have never seen or heard of any instances when he acted as a different human character on Sesame Street.