I honestly believe that musicians lift lines without even realizing it. When you reach the point that you've listened to so much music, parts of songs start coming to mind and you're killing yourself to try to remeraber where it originated from often with little success.
I have a personal experience with having lifted something. My story goes like this: it was 2005 and I hadn't listened to Nirvana probably in about 12 years. I was writing a song and I came up with what I thought was this really great opening guitar line. Only late last year did I realize that it wasn't original after listening to In Utero and hearing Heart Shaped Box again. Bingo, that's where I had got it from - my line was not identical but very very close. It was lodged deep somewhere in my mind so much so that it could spontaneously occur to me such that I could deceive myself into thinking I created it.
Paul McCartney interestingly was worried about the exact same thing when he came up with Yesterday. He recalled composing the melody in a dream, and playing it immediately after waking up in order not to forget it. But due to this, he was worried for months that he'd unconsciously plagiarized it. He played it to people all over the record industry to make sure that it wasn't the case, and finally, since nobody claimed it, he kept it for himself.
Could happen to any musician and I bet it frequently does. I come across curiously similar melodies all the time, especially recently after listening to some old Beatles albums again. I was listening to "Help" 1965, and I came across the song
It's Only Love. The melody of the chorus is remarkably similar to part of Grizzly Bear's
He Hit Me from last year. I'd bet my bottom dollar the songwriter got the melody from having heard that Beatles song at some point in his life.
Verdict: probably accidental, even in really suspicious cases.
EDIT: Well, except Led Zep - that's clearly blatant rip-off-ahoy!
