When did the church fall into an unrecoverable errror?

While there were many errors that marked the early church before the year 786 or 787, the last council that was agreed to by both the Roman Church and the Constantinople Church--today the Roman Catholic and Orthodox Churches--was the seventh council, which was forced to meet in Nicea to do its dirty deed.

That deed was to reverse an earlier council that rejected the use of icons or statutes because they could cause idolatry. Finally, a woman or queen mother came along who would help apostate bishops reverse this blessed stand against icons--flat pictures--by calling a new council. Though it had to move from one location to another to work its darkness, it managed to accomplish the adoption of paintings and iconography and drive these churches into the darkness of idolatry that remains to this day, despite their supposed distinction between worship and honor of icons.

In addition, the Roman church started allowing the honoring of statues (sculpture in the round) which was rejected even by this apostate seventh council.

In fact, a promotion on a show about icons on EWTN recently featured a priest who called icons the "vicarious presence of God and his saints."  This means the paintings or photos or statues should be treated as though they are the God or saints they represent.

Here's more on the background of all this from wikipedia...

In 786, the council met in the Church of the Holy Apostles in Constantinople. However, soldiers in collusion with the opposition entered the church, and broke up the assembly.[2] As a result, the government resorted to a stratagem. Under the pretext of a campaign, the iconoclastic bodyguard was sent away from the capital — disarmed and disbanded.

The Second Council of Nicaea -- The council was again summoned to meet, this time in Nicaea, since Constantinople was still distrusted. The council assembled on September 24, 787. It numbered about 350 members; 308 bishops or their representatives signed. Tarasius presided,[3] and seven sittings were held in Nicaea.[4] Proof of the lawfulness of the veneration of icons was drawn from Exodus 25:19 sqq.; Numbers 7:89; Hebrews 9:5 sqq.; Ezekiel 41:18, and Genesis 31:34, but especially from a series of passages of the Church Fathers;[1] the authority of the latter was decisive.

Aya Sofya of Nicaea, where the Council took place; Iznik, Turkey. It was determined that "As the sacred and life-giving cross is everywhere set up as a symbol, so also should the images of Jesus Christ, the Virgin Mary, the holy angels, as well as those of the saints and other pious and holy men be embodied in the manufacture of sacred vessels, tapestries, vestments, etc., and exhibited on the walls of churches, in the homes, and in all conspicuous places, by the roadside and everywhere, to be revered by all who might see them. For the more they are contemplated, the more they move to fervent memory of their prototypes.

Therefore, it is proper to accord to them a fervent and reverent adoration, not, however, the veritable worship which, according to our faith, belongs to the Divine Being alone — for the honor accorded to the image passes over to its prototype, and whoever adores the image adores in it the reality of what is there represented." St. Basil the Great The clear distinction between the adoration offered to God, and that accorded to the images may well be looked upon as a result of the iconoclastic reform.

However sculpture in the round (statues) was condemned as "sensual". The twenty-two canons[5] drawn up in Constantinople also served ecclesiastical reform. Careful maintenance of the ordinances of the earlier councils, knowledge of the scriptures on the part of the clergy, and care for Christian conduct are required, and the desire for a renewal of ecclesiastical life is awakened.

The papal legates voiced their approval of the restoration of the veneration of icons in no uncertain terms, and the patriarch sent a full account of the proceedings of the council to Pope Adrian I, who had it translated (the translation Anastasius later replaced with a better one).

This council is celebrated in the Eastern Orthodox Church as "The Sunday of the Triumph of Orthodoxy" each year on the first Sunday of Great Lent—the fast that leads up to Pascha (Easter)—and again on the Sunday closest to October 11 (the Sunday on or after October 8). The former celebration commemorates the council as the culmination of the Church's battles against heresy, while the latter commemorates the council itself.