[FactQuest]

Not being Catholic (don't look surprised, lol) I can't be very specific about Vatican II. I'm obviously blurring things around a great deal from ignorance. Thank you for taking my ramblings and seeing the point of the question - do some infallible teaching contradict other infallible teachings? Admittedly, without specifics, I can't argue the point very well, and am destined to lose this debate unless I do more research. And maybe even then. Speaking in general terms, it still seems unlikely that significant and controversial change could ever occur without some infallible teaching not contradicting another infallible teaching. Again, I can see how specifics might bear out your contention, but from a high level, it seems improbable.

[Campion]

Speaking in general terms, it still seems unlikely that significant and controversial change could ever occur without some infallible teaching not contradicting another infallible teaching.

I think you're assuming that practically everything is defined infallibly. In practice, very little is.

Vatican I laid out 4 criteria for an infallible Papal statement:

1. It must state a doctrine concerning faith and morals. (Matters of practice or discipline don't qualify.)
2. It must be teach that doctrine definitively. (Merely suggesting it, or conceding that it can be taught, doesn't qualify.)
3. It must be addressed to the whole church.
4. The Pope must be teaching in his capacity as supreme shepherd, not, for example, expressing his personal opinion, or merely his opinion as the Bishop of Rome.

It takes a certain amount of effort to meet all 4 criteria.