According to Andy, this is what happened one night recently when Conan's guests were actor James Spader and director Quentin Tarantino.
Tarantino tells Conan O'Brien about an up-and-coming Muppets version of The Wizard of Oz, and he talks about his favorite puppets, one of which (and I didn't catch the name) he described as "Joni Mitchell-looking." He proceeded to do some funky and idiotic impression by waving his head stupidly about. I was excited and rushed immediately to the computer to report it to JMDL. Thanks, Andy!