These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Covered Up: "El Condor Pasa (If I Could)", which is just Simon's lyrics sung over the Los Incas version of the Peruvian folk song "El Cóndor Pasa" by Daniel Alomía Robles. Similarly, "Scarborough Fair/Canticle" adds new lyrics sung in counterpoint to the original folk ballad.
Robles' son sued Simon and Garfunkel in 1970 to gain a writer's credit for "El Cóndor Pasa", but noted that the court case itself was friendly since the oversight was the result of Simon being misled to believe it was a traditional folk song, and has stated he bears Simon no ill will. Los Incas similarly did not mind the fact that their instrumental backing had been used without permission, and even became friends with Simon and had him produce their first English-language album.