One obvious example is opt-out system for organ donations. Who can honestly say it would have worse results than either forced organ donations, or opt-in?

Those who are against easy organ donation often argue that it would provide incentives for doctors to strive less to save people in accidents or suffering from issues like brain tumors, since on strict utilitarian grounds, that person's death might save several others. This isn't something you know is happening except statistically, which means no redress for those (or their heirs) so affected. Lots of people don't want to feel as though they cannot trust their doctor, even if more lives might be saved overall.