I once had the realization that a riff from Metallica's "The Four Horseman" was the same as that from Tom Petty's "Last Dance With Mary Jane" except, as I learned several years after the realization, it is actually the riff from "Sweet Home Alabama" which was introduced in a session as a joke by the guitarist to make fun of the drummer for having narrow musical knowledge. He was correct in his assumption that the drummer wouldn't recognize it, and it was included in the final version of the song.
The story: https://youtube.com/shorts/nY6CPOtN47w?si=K_9EKvKGbgnuGKSg
The riffs: https://m.youtube.com/watch?v=2V6HbSkAD5g&pp=ygUgZm91ciBob3J...
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