How can common believers be sure that the Church's decisions are correct and not erroneous? 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. So, the question is: How can common believers be sure that the Church's decisions are correct and not erroneous? Has this matter ever been discussed in history?
The simple answer is know your Bible.  God has given us His Word to guide us.  The church, being filled with sinful men, is going to err and has erred.  But God's word is 'settled in the heavens', is perfect, is truth (John 17:17).  So the primary source for truth must be God's words and not men's words.

It is however difficult sometimes to go against the church and her teaching.  The leaders of any given church have usually studied the Bible a lot more than the average Christian.  But we must also remember that God guides His children into truth, so study your Bible, know it, and then ask God for His Holy Spirit to guide you in all truth.