There have been songwriters who borrow
parts of songs and get away with it. Other cases are worthy of court cases and suing.
Borrowing:
Beatles —Birthday
from: Just a Little Bit, Roscoe Gordon
Beatles—All You Need is Love
from Chanson d’Amour, Fontaine sisters
(the da, da, da, da, da after all you need is love)
Rod Stewart, Killing of Georgie, “oh Georgie stay” part
From: Don’t Let Me Down, Beatles
Lennon: “The lawyers were asleep on that one”
Was sued. Outcome: Harrison was found to have subconsciously plagiarised the song, a verdict that had repercussions throughout the music industry. Rather than the Chiffons song, he said he used the out-of-copyright Christian hymn "Oh Happy Day" as his inspiration for the melody.