Protestants don't claim that they themselves are infallible. They know that no man is infallible. However, they do often claim that the scriptures are infallible.
If you believe the Pope is infallible because he has the Holy Spirit, I would assume you would believe the same about the scriptures which are "God-breathed".
If scripture is infalliable, and apparently, self-evidently true, why is there disagreement over the canon?