If it is a Required column header, it will fail validation. If it is an Optional column header, it will pass validation and the data import will not recognize it.
Scenario: In the Events data import, there is a field to identify where the event will take place. The column header for that field is "location". If it is misspelled ("locaiton") in the data import file, then that entire column for all records will be ignored during the import process. End result will be that none of the imported Events will have their location populated and you cannot use data import to update existing events.