Then -- or should I say "now"? -- there's the way modern mass media and instant communication can 'change the lyrics or tune' to a song almost overnight. E.g., last night on the NBC Christmas tree lighting celebration, Lionel Richie modified the lyrics to "Joy To The World," and I'll be surprised if some of those changes don't get echoed all over the internet and in singalongs in short order -- just because TV reaches so many people so quickly.
I have often seen such things happen with other Christmas carols and commonly sung songs.   It's like "the folk process" only at warp speed.

A similar thing happens with authorship attribution,   E.g., while the origin, date, and authorship of "Stille Nacht" is clearly documented and easy to find, all it seems to take is a few books or CDs that list it as "traditional" before millions of people come to accept that error as fact.