2 different type of answers to a same question
It would be necessary to allow two different type of answers for a same question, e.g. check answers and then, if the proposed answers doesn't apply, give an option "Other:...." where the person filling in the form would be able to enter his own answer.
Amy Lawler commented
If you add an question type of Choice, you can toggle the Multiple answers selection below the answers and the radio buttons change to checkboxes which allow more than one answer selection. The same applies to an add'l answer option of "Other".
This capability already exists! You can toggle on the "multiple answers" button while creating a question so that the person can select more than one answer. And you can also select 'Add 'Other' Option" while creating a question that let's them choose "other" and type in their own response.