Invalid content errors occur when saved HTML in the database doesn’t match what the save function currently generates. WordPress validates block integrity by comparing stored content with expected output.
This validation prevents silent corruption but creates errors when you modify save functions during development. Changing HTML structure breaks existing blocks in the database that use the old format.
Avoid “attempt block recovery” for development issues since it only fixes single instances. Instead, remove and re-add blocks after save function changes to ensure database content matches current expectations.