Reading through Liccione's circuitous description of his denomination's confusing process of self-discovery, he makes Mother Church sound like a patient suffering from amnesia. The poor dear is trying ever so hard to remember who she is:
The notion that the teaching authority of the Church is infallible under some conditions is certainly a development arising from the Church's early sense of her own indefectibility. Explicit use of the term 'infallible' predates Vatican I by centuries. It appears explicitly in Thomas Aquinas: "Whoever does not adhere to the teaching of the Church as to an infallible and divine rule can affirm what is "of faith," but he does not do so "by faith" (ST IIa IIae Q5 A3 resp). Around that time, both conciliar and papal infallibility were generally affirmed by theologians, though the relationship between the two forms of infallibility, as well as both to that of the Church in general, was temporarily obscured by the scandalous papal schism of 1378-1417 and some of the claims made by the Council of Constance in the course of resolving it. By the time of the Counter-Reformation, however, conciliar and papal infallibility were being taught together by the college of bishops as a whole. That's because conciliar dogmas could not be said to be binding unless ratified by the pope, and the papacy could not function as the locus of final appeal if a pope's ex cathedra ratifications were themselves subject to reversal as false. Thus, infallibility in all three forms--that of the Church as a whole, that of general councils, and that of the pope--were infallibly taught by the OUM over time.
It is not important to fix the precise times when one could say, from a scholarly standpoint, that the conditions had been met for such doctrines to have been infallibly taught by the OUM alone. If the doctrines in question are de fide, which they are, then something logically equivalent to them was always taught infallibly by the OUM; if that were not the case, then substantive addition to the deposit of faith would be occurring over time, which nobody is willing to allow. The "development" consists in coming to see this over time, when it was not fully explicit at first to those who were in fact exercising the charism of infallibility.
Limbo is often cited as a counterexample to LG's doctrine of the IOUM. But the problem has a standard resolution favored by the Pope himself. Limbo was a theory introduced in the Middle Ages to mitigate Augustine's view that unbaptized infants went permanently to hell--albeit with the "mildest of punishments"--and St. Thomas' version of it did not take long to gain general acceptance, which persisted until Vatican II. But limbo cannot be said to be de fide, precisely because it was introduced to mitigate the consequences of an Augustinian theory that was itself not de fide: the theory that original sin is personal culpa not just reatum, which latter term is weaker, and was used by Trent. That underlying Augustinian theory has been rejected by the Church (cf. CCC 405).
To summarize the essential points made by then-Cardinal Ratzinger, a given doctrine D counts as having been infallibly set forth by the OUM just in case its subject matter belongs to the deposit of faith, and it has been taught by the diachronic consensus of the college from the beginning. In his Doctrinal Commentary on Ad Tuendam Fidem, Ratzinger noted that such doctrines require "definitive assent" from Catholics. But of course, if we just left it up to scholars to decide when D satisfies those criteria, then requiring "definitive assent" would be meaningless; for scholars rarely agree for long if at all, and they are not the Magisterium anyhow. So the question whether D satisfies the relevant criteria, when that is a matter of dispute, ultimately must be answered by the Magisterium itself. Scholarly considerations are of course quite relevant too, but they are not in themselves decisive.
As I've already implied, the entire deposit of faith has been infallibly taught by the OUM from the beginning. If the dogmatic pronouncements of the infallible "extraordinary" magisterium were always necessary for the exercise of infallibility, then nothing was taught infallibly before the first ecumenical council--a consequence unacceptable for all sorts of reasons, one of which is that it is ultimately incompatible with the very idea of a "faith once delivered" in its entirety. And so, e.g., the assertoric content of the confessions of faith contained in "the Apostles' Creed" were infallibly taught by the OUM all along. Nobody disputes that; the question always is what the relevant affirmations mean, exactly; and such questions are settled over time by the Magisterium.