In "The Award" (Season 5 Episode 14), Brick Heck states, after getting a new suit and faced with the possibility of not going to an event, that his whole life he had "looked like Bert and/or Ernie." This refers to Brick's horizontally- and vertically-striped shirts.
In "Homecoming II: The Tailgate" (Season 7 Episode 7), Sean Donahue appears with longer hair and a beard, to which his mother Nancy Donahue apologetically explains to Frankie Heck, "He's just trying it out; doesn't mean anything. Jesus had a beard. And Jim Henson. And none of them were doing drugs, I can tell you that!"
In "The Core Group" (Season 8 Episode 1), Axl Heck tells his brother Brick "That haircut makes you look like a Muppet! Not a main one. Like one that played a waiter."
In "Ovary and Out" (Season 8 Episode 13), as Frankie and Mike Heck reminisce about their oldest son's inability to fall asleep when he was little, Frankie begins: "Axl was the worst, remember? Oh! We had to drive with him for hours. And listen to that Sesame Street cassette over and over." Mike adds: "But we had to skip that song with the Count." Frankie: "Oh yes, he was terrified of the Count! Aww!" Mike: "I always liked Oscar. The guy just liked trash, and didn't want other people messin' with it. Now why's that make him a grouch?" A short while later, Frankie starts crying: "It's just one minute, you're driving around with your kids, listening to Elmo sing "I Don't Want to Live on the Moon," and the next minute, your doctor's telling you he can't find your ovaries." After consoling his wife, Mike corrects her by adding: "I think Ernie sang "I Don't Want to Live on the Moon!" Frankie: "No. Ernie? Was it Ernie?" Mike: "I think it was." Frankie: "What'd I say?" Mike: "Elmo." Frankie laughs: "Elmo."