Happens to me often; e.g. my, "Now is the Cool of the Day" was recently 'collected' as an old spritual and a choral arrangement made and marketed at music conferences...I think the arranger still believes that it was an old spritual!

Another e.g. my, "The L&N Don't Stop Here Anymore" has been mistaken many times as trad. Some have even recorded and claimed to have 'helped to write it,'namely one Bill Phillips of Black Mountain, NC.