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Appearances by the Muppets, where show titles and dates are unknown. When information surfaces, these items will be moved to their own pages.
Ernie and Bert with glasses of milk
This is in the first episode of CNN's 2015 documentary series The Seventies (on Netflix as of April 2016). I upload it as a gif to show the shot they're using, which wouldn't have been used on Sesame Street; the lighting also kind of gives that away. Speculation: something for The Ad Council or similar government-funded PSA? A PBS interstitial?
Bert, saying "Ernie"
A couple seconds appears in A&E Biography: Sesame Street when Frank is talking about his characters.
Muppet Babies playhouse
Template:Medialink appears in Child's Play. We don't know anything about it, but the movie was released in 1988. Scott has had an ebay watch enabled since April 2014, but nothing's shown up.
Kermit in special with Phylicia Rashad
YouTube video c.April 2010 (now dead, but Scott may have this archived) featuring Phylicia Rashad singing French and dancing to disco. The first few frames of the video showed Kermit wearing a tux, sitting at a table with food in front of him before it cut to Rashad's sequence.
Pau Hana Years
Big Bird and Oscar appear in the 1000th episode of this show in 1977. This was probably filmed during the Sesame Street cast's visit to Hawaii.
A video filmed by one of the show's producers documenting the production of the episode can be found here: http://vimeo.com/10540022
Jim and Kermit on NBC news show
Some footage of an appearance by Jim Henson and Kermit on an NBC show was included in the Kevin Clash appearance on The Big Idea.
Promotional Disney Appearance
Gonzo and Scooter appeared in a 1991 appearance on a UK morning show promoting Walt Disney World and its Disney-MGM Studios park attractions. They told fat jokes about Miss Piggy. (YouTube)
Japanese Appearance
Jim Henson's Red Book has information on a guest appearance in a 1976 Japanese special starring the Tokyo-based pop star Goro Noguchi, but it doesn't reveal the title (Henson noted it in his journal simply as "Goro Special". The Muppets were invited by Nippon Television to appear on the program and the characters performed for about twenty minutes in sketches and songs throughout the ninety-minute special. Performers included Jim Henson, Frank Oz, Jerry Nelson, and Dave Goelz (along with assistance from Jane, Cheryl and Lisa Henson).
1983 Kennedy Center Tribute
Someone on Muppet Central Forums found this video, but doesn't appear to have a page. (YouTube)