Big one happened in the movie biz. "The Sting" music by Marvin Hamlisch should have been music by "Scott Joplin" adapted and arranged by Hamlisch.
I wonder about "Black Is The Color of my True Love's Hair". Jean Ritchie sings a myxolydian version of it but Niles claimed he wrote it. His is in a minor key.
A.P. Carter did not write "Wildwood Flower". 1860 by Maud Irving and J.P. (?)
When you get to the tunes:
"God on Our Side" (Patriot Game) (earlier versions of One Morning In May)
"Masters of War (Notamun Town)
"Roll On Columbia" (Irene Goodnight)
"This Land is Your Land" (Little Darlin' Pal of Mine but earlier hymn version)
"Smetana's Moldau" (Hatikva)
"Pastures of Plenty" (Pretty Polly)
"I Ain't Got No Home in This World Anymore (Jesse James) and an earlier hymn.
"Little Boxes" (There's a Pawn Shop On the Corner in Pittsburgh Pennsylvania)
"Angeline The Baker" (Angelina Baker by Stephen C. Foster)
"Done Laid Around, Done Stayed Around" (I Got a Home in GloryLand) and (Do Lord Deliver Me)
Then there's the problem of Tom Dooley. Frank Profitt did not write it but successfully sued the KT over it.
"12th of Never" (Riddle Song)
"Delta Dawn" (Come and Go With Me To That Land)
Maybe some more ideas....much of the tunes were in public domain but I bet they had composers some where.