Ecclesiastes 10:1 "Dead flies make the perfumer's ointment give off an evil odor; so a little folly outweighs wisdom and honor."

Merriam-Webster has several definitions for folly, but the first two are "lack of good sense or normal prudence and foresight" (a/k/a, a lack of virtue), and "criminally or tragically foolish actions or conduct." So we are not talking about mistakes, even stupid mistakes, here. We are talking about someone who might seem virtuous, but has not in fact developed proper prudence, or is ignoring it and causing harm.

I think the sense of this verse is what Jesus was getting at when He said "be perfect as your heavenly Father is perfect." (Matthew 5:48) He of course knows we cannot be as perfect as the Father, we are fallen. But we can try, and we can orient our lives, our thoughts, our words, and our actions toward this goal. If we do, we will grow in virtue and, although we still may make mistakes, they will not be "folly". 

Our good actions are not placed on a scale and weighed against our sinful or self-indulgent ones. Rather, the intention and moral consideration with which we are living our lives are most important. What we choose to do speaks to whether we are on the right track. If we are willing to engage in "folly" even once, especially if we think our storehouse of wisdom and honor will offset it, we are showing ourselves not to be truly wise or honorable in the first place. These foolish choices become as flies in the perfume of our good works, ruining the whole lot by calling into question whether the good works were actually done with true virtue or honor in the first place.

The sinner who wants and intends to do good, but keeps falling and asking for forgiveness does not pollute his good works with folly. But someone who reveals a lack of virtue through folly, or supposes his willful foolishness will be forgotten due to his reputation for wisdom and honor, makes an eroor that is hard to rectify.