There's a Muppet big/bigger/biggest clip that hasn't been seen in decades, but Frank really outdid himself in scaring me to death.
There were three monsters, two of whom we've seen before (one was green, the other was orange- in the skit where Herry tells Grover the alphabet, and asks him to keep it a secret, Grover blabs to these two monsters, called Pamela and Fenwick)
Green Monster (Richard Hunt): "Big!"
Orange Monster (Jerry Nelson): "Bigger!"
Frank has a cute little pink monster (Billy, according to MuppetWiki) who starts growing to the point where he fills up the entire screen. He looks down at the other two who are now shaking in their boots and yells in his deepest Cookie Monster voice:
"BIIIIIIIGGGGGGGGGGGGGESSSSSSSSTTTTTTT!!!!!!!"
The other two run off, and Billy turns to look at us, like he's gonna scare us next.
Thanks for the nightmares, Frank. I owe you one.