Only when pronouncing on matters of faith and morals, and only when speaking ex cathedra - which means "from the chair" (of Peter), but in doctrine it's defined as speaking in the context of a gathering of cardinals and top theologians who've debated the issue.
When I was a child* the idea got into my head that it wasn't anything to do with a claim of personal infallibility, only that if he botched it under those conditions he'd be struck dead mid-pronouncement just before getting to the troublesome part. Then they'd all look around and say "Welp, I guess everything he said up to that point was good. Now who do we promote to finish the sentence?" Then they'd assemble the famed College of Cardinals for another of their historic keggers.
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When I was a child* the idea got into my head that it wasn't anything to do with a claim of personal infallibility, only that if he botched it under those conditions he'd be struck dead mid-pronouncement just before getting to the troublesome part. Then they'd all look around and say "Welp, I guess everything he said up to that point was good. Now who do we promote to finish the sentence?" Then they'd assemble the famed College of Cardinals for another of their historic keggers.
*mid-thirties