I would say so! Any time God speaks - through men with written word or through the leading of the Holy Spirit within our hearts - it is infallible.
prophets are not infallible in general?
No. And not all prophecy will come true! That is, in fact, Biblical. Some of the greatest prophets of the Bible were in fact quite fallible. And not all prophecies have come to be.
That is are prophets normal men who, on occasion under the direction of God, speak his word infallibly?
I would say so, yes. But it is God that gives them the infallible nature, and it is not pre-ordained by their position in life. And it will be the body of Christ as a whole - the laity and ordained - who will show their words to be from the Holy Spirit, from God.
God calling you and speaking through you makes you infallible, and it can be through a milkmaid or the Pope. However, by virtue of being Pope you do not have the guarantee that God will speak through you any more than He will through the milkmaid.
And the words of the prophet must be tested against Scripture and against the leading of the Holy Spirit of the entire church at large. Much of the early church as recorded in Acts took action based upon consensus of the entire congregation, believing in the leading by the Holy Spirit of all assembled.
“And the words of the prophet must be tested against Scripture...”
“13 Now that same day two of them were going to a village called Emmaus, about seven miles from Jerusalem. 14 They were talking with each other about everything that had happened. 15 As they talked and discussed these things with each other, Jesus himself came up and walked along with them...25 He said to them, “How foolish you are, and how slow of heart to believe all that the prophets have spoken! 26 Did not the Christ have to suffer these things and then enter his glory?” 27 And beginning with Moses and all the Prophets, he explained to them what was said in all the Scriptures concerning himself.”
Jesus, immediately after His resurrection, used Scripture for authority. I suppose he could have said, “Dimwits! I just rose from the dead!”
But instead, He used scripture. He showed that what He experienced was true by scripture.
If Jesus used Scripture to prove His words, then we obviously should use scriptures to prove - test - the words of men. And the Popes fall terribly short.