The first time I heard of this was 30 years ago -- a friend of mine had doubts about Paul's reliability.
One evening, after a service, we asked a pastor about the differences between the Gospels and the Pauline letters and he told us that, in case of conflict, the teachings of Paul take precedence over those of the Lord Jesus because Paul was the apostle to the Gentiles. His explanation sounded weak (or incomplete), but we accepted it.
It wasn't until some six months ago that I found the full answer to our question: While on earth the Lord Jesus was reinterpreting the Mosaic Law, and after the cross He preached a different way of salvation through the apostle Paul. In other words, Jesus' earthly ministry was under the dispensation of the Law and Paul's ministry was under the dispensation of grace. This is the reason why there are differences between the Lord Jesus and the apostle Paul.