To my surprise, I recently learned that a friend had no idea about the controversy surrounding the song 'Scarborough Fair', as performed by Simon & Garfunkel. It's an interesting piece of music trivia. Wikipedia describes it like this.
Paul Simon learned the song in London in 1965 from Martin Carthy, who had picked up the tune from the songbook by Ewan MacColl and Peggy Seeger and included it on his eponymous 1965 album. Simon & Garfunkel set it in counterpoint with "Canticle" – a reworking of the lyrics from Simon's 1963 anti-war song, "The Side of a Hill", set to a new melody composed mainly by Art Garfunkel. It was the lead track of the 1966 album Parsley, Sage, Rosemary and Thyme, and was released as a single after being featured on the soundtrack to The Graduate in 1968. The copyright credited only Simon and Garfunkel as the authors, causing ill-feeling on the part of Carthy, who felt the "traditional" source should have been credited. This rift remained until Simon invited Carthy to perform the song with him as a duet at a London concert in 2000.
I think it goes further than that. The dispute certainly aroused strong feelings in British folk music circles, where Martin Carthy's setting of 'Scarborough Fair' was widely regarded as the original, 'trad-folk' authentic one. Paul Simon was derided by some as having 'stolen' Carthy's arrangement without any acknowledgment or recompense. Carthy reportedly decided not to sue over the issue, but bad feeling remained for a very long time.
This morning, I'll let you judge for yourself. Here's Martin Carthy's 1965 version.
And here's Paul Simon's arrangement, from 1966.
From the sound alone, I reckon it's pretty much an open-and-shut case. Carthy's arrangement was undoubtedly used by Paul Simon, in a slightly adapted form. Why he never acknowledged that, or paid any sort of license fee, I have no idea.