You can tell if someone is singing the wrong words to "Streets of London" or "American Pie", because there is a single, correct version of those songs. It is not possible to sing the wrong words to a folk song, because there is no correct version.[quote]
Howard,no,that only applies to a traditional Folksong,not a contemporary folksong,the writer of the song has every right[IF HE /SHE WISHES] to turn round and say,no, you are singing the wrong word, which is altering the meaning,example Cyril Tawneys, chicken on a raft[NAVAL SLANG egg on toast],becomes meaningless if sung as Chicken on a Rat.
haul away the diso [daighso],[SammysBar] becomes meaningless,if you sing haul way the Dino,or haul away the tiso.
with contemporary folk song there are correct versions,   or perhaps you dont think Sammys Bar/ chicken on a raft are folk songs.