A story involving R. Asi and R. Zeira about eggs that were cooked on wine - did that combination carry a risk of "demai" (possibly untithed produce)? Where even R. Asi wasn't thinking about this concern - and how God prevents error by those who are righteous. Which leads us into the ongoing discussion of mixtures or combinations. Also, what about the concern of the population of those unschooled (am ha-aretz) in the halakhic arena of tithing? What if the person who is ignorant switches out that food, perhaps out of concern that it goes bad, or one's personal status of being impure/pure to be protective. That is, one's intent could be entirely on target, but the action is still problematic for a potentially demai mixture.