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We have discovered two novel types of planar defects that appear in heteroepitaxial YBa 2 Cu 3 O 7− δ (YBCO123) thin films, grown by pulsed-laser deposition (PLD) either with or without a La 2/3 Ca 1/3 MnO 3 (LCMO) overlayer, using the combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and electron energy loss spectroscopy (EELS) mapping for unambiguous identification. These planar lattice defects are based on the intergrowth of either a BaO plane between two CuO chains or multiple YO layers between two CuO 2 planes, resulting in non-stoichiometric layer sequences that could directly impact the high-T c superconductivity.