Well lattice-matched monolayer films of hexagonal boron nitride (h-BN) can be grown on Ni(111) surfaces, representing a nominally ideal interface for preparing ultimately thin metal-insulator-metal (MIM) structures. In a detailed study, combining local and non-local probes, the presence of characteristic defect lines is uncovered, and a model for their atomic structure is proposed. They have a strong influence on the growth morphologies of metal deposits. For room temperature deposition, they act as anchors for cluster nucleation, thus effectively short circuiting the MIM structure. For high-temperature deposition, the defects collect Co adatoms very efficiently and lead to Co intercalation underneath the h-BN film. [DOI: 10.1380/ejssnt.2003.124]