Complex sentence: Sciambra blamed himself for leaving out the Shaw/Ferrie/Oswald party episode in his memo, but it was an omission that Shaw's attorneys were able to exploit by raising questions about the validity of Russo's testimony.
Split into simpler sentences: Sciambra blamed himself for leaving out the Shaw/Ferrie/Oswald party episode in his memo. It was an omission that Shaw's attorneys were able to exploit by raising questions about the validity of Russo's testimony.