Professor Ehrman makes a good point below. As a committed evangelical Christian I wrestled with the problem of Paul’s error for a very long time. But Paul was just a man who got some things wrong and there is nothing odd or surprising about that. Unfortunately, his occasional letters to churches have acquired the status of inerrant Word of God and Holy Scripture, and for many Christians this inevitably creates a massive problem.
Ehrman published this on his Facebook page a while ago. The spelling is pretty dodgy..