  YFeO$_3$ thin films are a recent addition to the family of multiferroic
orthoferrites where Y\textsubscript{Fe} antisite defects and strain have been
shown to introduce polar displacements while retaining magnetic properties.
Complete control of the multiferroic properties, however, necessitates
knowledge of the defects present and their potential role in modifying
behavior. Here, we report the structure and chemistry of antiphase boundaries
in multiferroic YFeO$_3$ thin films using aberration corrected scanning
transmission electron microscopy combined with atomic resolution energy
dispersive X-ray spectroscopy. We find that Fe\textsubscript{Y} antisites,
which are not stable in the film bulk, periodically arrange along antiphase
boundaries due to changes in the local environment. Using density functional
theory, we show that the antiphase boundaries are polar and bi-stable, where
the presence of Fe\textsubscript{Y} antisites significantly decreases the
switching barrier. These results highlight how planar defects, such as
antiphase boundaries, can stabilize point defects that would otherwise not be
expected to form within the structure.
