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: His Willie Nelson duet "Mamas Don't Let Your Babies Grow Up to Be Cowboys" was first released by Ed Bruce. Similarly, their "Good Hearted Woman" was originally a Top Ten single for Jennings solo, but the duet version was a #1.
"I'm a Ramblin' Man", originally written and recorded by Ray Pennington (who produced Waylon's version).
"Clyde", originally a J.J. Cale album track.
Also on the receiving end: he recorded "Where Corn Don't Grow" in 1990 but it only got to #67 on the country chart. 6 years later it became a Top 10 hit for Travis Tritt.
Epic Riff: "I'm a Ramblin' Man" and "Rainy Day Woman" are rare classic country examples of this; "Rainy Day Woman"'s riff is played on a steel guitar.