Contradiction with 1 Samuel 18:1
This verse describes an initial deep friendship and bond between David and Jonathan, which contrasts with the tension and potential distrust inferred in 1 Samuel 20:1 as David flees due to Saul's intent to kill him despite his bond with Jonathan.
1 Samuel 18:1: And it came to pass, when he had made an end of speaking unto Saul, that the soul of Jonathan was knit with the soul of David, and Jonathan loved him as his own soul.
Contradiction with 1 Samuel 19:6
This verse states that Saul heeded Jonathan's plea and vowed not to kill David, which contradicts David's fear and statement in 1 Samuel 20:1 about Saul seeking his life.
1 Samuel 19:6: And Saul hearkened unto the voice of Jonathan: and Saul sware, [As] the LORD liveth, he shall not be slain.