I've noticed when setting up surveys that sometimes the answers will be deleted before submission. For example, if a survey asks a user to select 6 choices and the user does not do this, upon submission the survey will delete the responses. Please see the below test survey to see this in action. The survey should indicate the error message but keep the responses so that the user does not lose their work.

http://nas.nasonline.org/site/Survey?ACTION_REQUIRED=URI_ACTION_USER_REQUESTS&SURVEY_ID=19641