In this 1972 clip from the film, The Complete Earl Scruggs Story, Joan Baez breaks into a funny imitation of Bob while singing It Ain't Me, Babe (at about 1:16)
The guitarist is a 16-year-old Randy Scruggs. The film includes interviews by Bill Monroe, Doc Watson, Charlie Daniels, The Byrds, and others. I haven't seen the film yet, it's on my short list.
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