Tunes (both instrumentals and the airs to songs) get processed a lot more than texts, and the processing usually passes unremarked. Hamish Imlach came up with his tune for "Black Is the Colour" when (probably in a drunken stupor) he couldn't remember the John Jacob Niles one properly. Most people in Scotland now sing it his way: it's a great improvement. And they also don't realize it's different from the original.

Then there's accidental plagiarism. Gordon Duncan coming up with "The Sleeping Tune" on waking up from a drunken coma at Lorient after hearing the "Kerfank 1871" an-dro the day before; Dick Gaughan composing his tune for "Both Sides the Tweed" by a subconscious shift of mode and tempo from "Rosin the Bow"; Phil Cunningham turning "She Was Poor But She Was Honest" into "Sarah's Waltz" by some associational process Sarah would probably prefer we didn't think about.

And virtually any instrumental dance tune is fair game for mutation. It's unlikely that any professional will take up an amateur-generated version as their own, but they'll always do the same sort of recomposition.