Contradiction with John 15:18
This verse speaks of the world's hatred for followers of Christ, which could imply that the visit would not be "not in vain" but fraught with opposition.
John 15:18: If the world hate you, ye know that it hated me before [it hated] you.
Contradiction with 1 Corinthians 2:3
Paul admits to being with the Corinthians in "weakness, and in fear, and in much trembling," which contrasts with the confident assertion in Thessalonians.
1 Corinthians 2:3: And I was with you in weakness, and in fear, and in much trembling.
Contradiction with Acts 17:5
Describes the turmoil and opposition Paul faced in Thessalonica, potentially contradicting his claim that their entry "was not in vain."
Acts 17:5: But the Jews which believed not, moved with envy, took unto them certain lewd fellows of the baser sort, and gathered a company, and set all the city on an uproar, and assaulted the house of Jason, and sought to bring them out to the people.
Contradiction with 2 Corinthians 11:24-26
Paul's recounting of his many sufferings and rejections can seem to counter the notion of a visit being wholly successful or fruitful.
2 Corinthians 11:24-26: Of the Jews five times received I forty [stripes] save one.
Contradiction with Galatians 1:10
This verse discusses seeking approval from God rather than humans, contrasting with the emphasis on the reception by humans in 1 Thessalonians 2:1.
Galatians 1:10: For do I now persuade men, or God? or do I seek to please men? for if I yet pleased men, I should not be the servant of Christ.