  We present optical photometry and spectroscopy of the optical transient SN
2011A. Our data spans 140 days after discovery including $BVRIu'g'r'i'z'$
photometry and 11 epochs of optical spectroscopy. Originally classified as a
type IIn supernova (SN IIn) due to the presence of narrow H$\alpha$ emission,
this object shows exceptional characteristics. Firstly, the light curve shows a
double plateau; a property only observed before in the impostor SN 1997bs.
Secondly, SN 2011A has a very low luminosity ($M_{V}=-15.72$), placing it
between normal luminous SNe IIn and SN impostors. Thirdly, SN 2011A shows low
velocity and high equivalent width absorption close to the sodium doublet,
which increases with time and is most likely of circumstellar origin. This
evolution is also accompanied by a change of line profile; when the absorption
becomes stronger, a P-Cygni profile appears. We discuss SN 2011A in the context
of interacting SNe IIn and SN impostors, which appears to confirm the
uniqueness of this transient. While we favour an impostor origin for SN 2011A,
we highlight the difficulty in differentiating between terminal and
non-terminal interacting transients.
