There is a difference between שגה (with a ה at the end) and שגג (with a repeating ג): שגה is a mistake in consideration, that he errs because of consideration or sense, etc. And שגג is an unintentional sin in deed. And this will be:

If through error in consideration, like eating prohibited fat that seems to him to be permissible to eat.

Or without consideration, for example when prohibited fat becomes confused for permissible fat.

השגיאה is attributed to the Sanhedrin, while השגגה is attributed to the masses.