Category and course grades can be overridden easily, but reverting to a calculated grade is not obvious for the average user. Having an 'overridable' option (like the Lock/unlock), default to false, for category and course grade items would prevent accidentally overriding.
If someone has a patch for this in 2.5 please link here!
at the moment the quickest way to get rid of 'accidental' overrides by teachers is:
UPDATE`mdl_grade_grades` LEFT JOIN `mdl_grade_items`
WHERE mdl_grade_items.courseid = 01 (courseid) in phpMyAdmin or equivalent.