I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel—not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.
For am I now seeking the approval of man, or of God? Or am I trying to please man? If I were still trying to please man, I would not be a servant of Christ.—Galatians 1:6-10 (ESV)
The bolded part above is odd. Leaving out the reference to "an angel", it reads: "if we should preach to you a gospel contrary to the one we preached to you, let him be accursed." Does Paul expect to change (or rather lose) his mind or is he expecting someone to forge a letter from him? The context of the letter seems to assert authority for himself, so the idea that he and his fellows might preach a contrary gospel seems to contradict his theme.