I am referring to a person's actions out of love for God and/or neighbor. Let's say you see someone living a few doors down from you who is hungry, and out of love for your neighbor you bring them some food that you made yourself, and tell them you made it from scratch, which is true. Let's say you included some artificial flavoring and coloring, though. The person eats your food, and gets really sick because he's allergic to artifical flavorings and colorings, plus allergic the the peanut oil you used to fry part of the food. He has to be taken by ambulance to the hospital, barely saving his life and costing him a lot of money that he doesn't have. You acted out of love but did harm to the person. Not on purpose, but the whole episode of your trying to help the person ended up causing a lot of harm. Would it be a sin to feed this food to the person if you KNEW about his allergies? Yes, without question in my mind. Is it a sin to do so but you don't know about his allergies? No, that is not a sin, it is a mistake. Once you realize your mistake, I think you are obligated to do something about it, it would be a sin to try to make excuses and blame the other person, etc.