But while quoting a song is often a common practice, it has been established that you cannot sing 90% of Howlin Wolf's “Killing Floor Blues”, throw in a Robert Johnson line about “squeeze my lemon till the juice runs down my leg”; and then call it “the Lemon Song” (Led Zeppelin)and not pay Howlin Wolf his money.
But the riff on squeezing lemons was obviously not “plagiarizing” Robert Johnson.
Thanks. I hadn't noticed it myself until someone pointed it out to me on a music forum.