Is one who followed a scholar who erred in his fatwa out of trust sinful, and what must be done to avoid falling into such temptations, especially with the difficulty of reaching scholars?

The first thing we advise is to strive diligently in seeking knowledge, for some of it is obligatory for every Muslim to know, such as the rulings of prayer and zakat and the like. And some of it is generally مطلوب (required to be known), and if some people know it, its obligation is lifted from the rest. The community should bring in scholars who teach people their religion. If an ordinary person asks a scholar who is trusted in his knowledge and piety, and then the scholar's fatwa turns out to be wrong, there is no blame on him, because he did what he was commanded to do. However, his duty is not to persist in the error after he comes to know the truth.