Contradiction with Matthew 28:20
Jesus promises to be with his followers "alway, even unto the end of the world," which contrasts with Paul's expression of being taken away from the Thessalonians in presence, but not in heart.
Matthew 28:20: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, [even] unto the end of the world. Amen.
Contradiction with Hebrews 13:5
God's promise of "I will never leave thee, nor forsake thee" contradicts the separation from the Thessalonians that Paul mentions.
Hebrews 13:5: [Let your] conversation [be] without covetousness; [and be] content with such things as ye have: for he hath said, I will never leave thee, nor forsake thee.
Contradiction with Romans 8:38-39
Paul's assertion that nothing can separate us from the love of God contradicts his own experience of being separated from the Thessalonians.
Romans 8:38-39: For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come,
Contradiction with Matthew 18:20
The assurance that where two or three are gathered in Christ's name, He is present, contrasts with Paul's experience of physical separation despite being united in spirit.
Matthew 18:20: For where two or three are gathered together in my name, there am I in the midst of them.