Grover attempts to surprise Ernie.
So I cheated again, because this one doesn't have either Bert or Ernie in it. But it does take place in their apartment, at least. Although that does raise the question of where Bert and Ernie are while this sketch is going on. Maybe they went on vacation and asked Herry to apartment-sit for them. And...told him to put on any clothes out of their wardrobe that he wanted...or something.
Maybe I should stop trying to apply logic to Sesame Street skits.
Grover doesn't act very surprised. I would have expected him to act much more surprised than that. And on close examination, "Ernie's" chair appears to be made out of cardboard.