When Rus went out into the field to pick up the Shikcha (dropped or forgotten crops which must be left for the poor), Boaz sees her great piety and tells his servants to drop more on purpose so that she can get more food.
This seems to be a problem as normally you are not required to separate Masseros (tithes) from Shikcha, but if one drops wheat on purpose, he will cause her and her family to eat un-tithed foods as the food will be obligated in masseros but she won't know it.