There have been some councils, the decisions of which were later reversed by some latter councils. Thus, if your position on a particular matter that was being considered at those councils was too strong, you could easily be excommunicated from the Church as a heretic between those two councils, while you would be just fine after the second one, or the other way around. The most recent example that comes to my mind is probably the incumbent Pope's decision to condone contraceptives.
Here is another example: the council of Hieria in 754 a.d. and the Second Council of Nicaea in 787 a.d., both of which appropriated their titles as Seventh Ecumenical Council. The first council forbade the use of icons, and the second one reversed that decree. I know that one could say here that the Synod of Hieria was not an Ecumenical Council (it was called and chaired by the imperial family and no patriarchs were present, plus, its status is implicit in the Second Council of Nicaea declaring itself to be the actual Seventh Ecumenical Council), however, it doesn't really matter. What matters is that at the time when it took place it was announced as an Ecumenical Council, which means that ordinary folks - at least during the time between those two councils - had to follow the decrees of the council of Hiernia, otherwise, they would definitely fall into the category of heretics according to the decree of that council.
So, the question is: How can common believers in the Roman Catholic Church be sure that the Church's decisions are correct and not erroneous?