I have a field for which the user may select multiple responses, and is likely to be implemented as a drop-down with checkboxes. The user can select one, two, or many from the list of options. We're even providing a "All options" option to speed things along.
The field has one last requirement though, and that is that the user must select at least one option. They can select and de-select each option independently, but they must leave at least one selected.
Normally with a form we can also do field-validation, but in this case the values selected are dynamically linked into a chart thing and so we're looking for a UI which effectively forces the constraint.