We're running Questionnaire version 2008060402, which I know isn't the most current version, but I didn't see this bug posted anywhere, so I'm not sure it's been fixed in the current version.
When a questionnaire uses the question type "Rate (scale 1..5)" and includes an "NA" column, if a user tries to submit the questionnaire without answering all of the questions, the questionnaire gives them a warning and prompts them for those answers. However, it also changes all of their NA rankings to Rank 1!! This is a terrible bug, particularly because we are using the Questionnaire for elections.
As a workaround, we are getting rid of the NA column and adding an "Unranked" column, which we will cull out of the final results. This is not ideal, however, since it means we'll need to massage the data outside of Moodle to get final results.