Iconoclasm. It was basically a final assault by the devil to bring back all the implications of the previous major heresies (Arianism, Macedonianism, Nestorianism, Monophysitism), fueled by a worldly attempt to please Muslims, and it destroyed countless valuable artefacts. It remained a problem a century after the 7th ecumenical council. And Protestantism revived it strongly, and today it is rather common (even if not as extreme as in the 8th-9th centuries).