Contradiction with Matthew 10:5-6
Jesus initially instructed the disciples not to go to the Gentiles, but only to the lost sheep of Israel, which contradicts the universal mission implied in John 17:18.
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 Matthew 15:24
Jesus states that He was only sent to the lost sheep of Israel, which contrasts with the broader sending in John 17:18.
Matthew 15:24: But he answered and said, I am not sent but unto the lost sheep of the house of Israel.
Contradiction with Acts 10:28
Peter initially acknowledges that it is unlawful for a Jew to associate with a Gentile, contrasting with the idea of being sent into the world as stated in John 17:18.
Acts 10:28: And he said unto them, Ye know how that it is an unlawful thing for a man that is a Jew to keep company, or come unto one of another nation; but God hath shewed me that I should not call any man common or unclean.
Contradiction with Galatians 2:7-9
Paul and Peter have different missions; Paul to the Gentiles and Peter to the Jews, which shows a distinction in mission rather than the unified sending implied in John 17:18.
Galatians 2:7-9: But contrariwise, when they saw that the gospel of the uncircumcision was committed unto me, as [the gospel] of the circumcision [was] unto Peter;
Contradiction with Romans 15:20
Paul expresses a desire not to build on someone else's foundation, suggesting a mission strategy potentially at odds with the sending into the world described in John 17:18.
Romans 15:20: Yea, so have I strived to preach the gospel, not where Christ was named, lest I should build upon another man's foundation:
Paradox #1
It might be seen as inconsistent because Jesus is sending his followers into the world with a mission, but elsewhere the Bible talks about being separate from the world and its corruptions. This could seem like a conflict between engaging with the world and staying apart from it.