SS/EC crossovers
Other Sesame Street / Electric Company crossovers that I'd heard about, but don't remember exactly:
-- Big Bird once visited Fargo North in his shop.
-- some of the cartoons, like the 2001: A Space Odyssey monolith that crumbles to reveal "ALK" or some other letter combination, to the sound of "Also Sprach Zarathustra", appeared on both shows.
-- supposedly, one day Rita Moreno wandered on to Sesame Street and called out "Hey You Guys!". The adults politely told her that she was on the wrong show.
Then there was this routine I saw in which Big Bird came into Hooper's store, and Mr. Hooper was paying his bills. Mr. Looper explained to BB that if he didn't pay such-and-such a bill, he would lose a specific amenity.
HOOPER: "And this bill is for the Electric Company. If I don't pay this bill"
(entire set goes dark).
I wouldn't be surprised if Big Bird responded "The Electric Company? Boy, that's a great show."