Contradiction with 1 Corinthians 15:9
This verse has Paul declaring himself the least of the apostles, which may seem contrary to his affirmation in 2 Timothy 1:1 where he is confidently declaring his apostleship.
1 Corinthians 15:9: For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God.
Contradiction with Acts 1:21-22
These verses explain criteria for apostleship which are based on being with Jesus during his earthly ministry and witnessing the resurrection, which could contradict Paul's later apostleship as stated in 2 Timothy 1:1 since he was not present during Jesus' ministry.
Acts 1:21-22: Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us,
Contradiction with Galatians 1:1
Although this verse does not directly contradict, it supports the legitimacy of Paul’s apostleship being challenged, contrary to the confident assertion in 2 Timothy 1:1.
Galatians 1:1: Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead;)