Contradiction with Matthew 10:5-6
Jesus instructed the apostles not to go to the Gentiles or Samaritans, but only to the lost sheep of Israel, which contradicts Paul and Barnabas speaking to both Jews and Greeks in Acts 14:1.
Matthew 10:5-6: These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into [any] city of the Samaritans enter ye not:
Contradiction with Romans 11:13
Paul refers to himself as the apostle to the Gentiles, whereas Acts 14:1 describes him preaching to both Jews and Greeks, which could imply a contradiction in his primary mission focus.
Romans 11:13: For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, I magnify mine office: