One of the most enjoyable few minutes I ever enjoyed in a folk club was at the Empress of Russia, in Islington, in the late 70s with Flowers and Frolics. No hangups with this crowd! Nick Havell, the trombonist, and an opera buff, stood up and sang a stirring version of 'La Donna e mobile.
Backed by the band -melodeon, anglo concertiona, banjo, drums & then at the end, Nick sat down and joined in with the trombone for the last run-through.
I don't know if it was folk music or opera, but it was bloody good fun!