  Some reports of supernova (SN) discoveries turn out not to be true
core-collapse explosions. One such case was SN 2009ip, which was recognized to
be a luminous blue variable (LBV) eruption. This source had a massive hot
progenitor star identified in pre-explosion data, it had documented evidence of
pre-outburst variability, and it was subsequently discovered to have a 2nd
outburst in 2010. This same source rebrightened again in 2012, and early
spectra showed the same narrow-line profiles as before, suggesting another
LBV-like eruption. We present new photometry and spectroscopy of SN 2009ip,
indicating that it has transitioned into a true SN. The most striking discovery
in these data is that unlike previous reports, the spectrum exhibited Balmer
lines with very broad P-Cygni profiles characteristic of normal Type II
supernovae (SNe II), in addition to narrow emission lines seen in SNe IIn and
LBVs. Emission components have FWHM 8000 km/s, while the P-Cygni absorption
component has blue wings extending to -13,000 km/s. These velocities are
typical of SNe II, but have never been associated with emission lines from a
nonterminal LBV-like eruption. Initially, the peak absolute magnitude seemed
fainter than that of normal SNe. However, after a brief period of fading, the
source quickly brightened again to M_R=-17.5 mag over a couple days. The broad
lines mostly disappeared, and the spectrum began to resemble the early
optically thick phases of SNe IIn. Two weeks later the source leveled off near
-18 mag, after which broad emission lines again developed in the spectrum as
the source faded. We conclude that the 2012 outburst of SN 2009ip was the
result of a true core-collapse SN IIn that occured when the progenitor star was
in an LBV-like outburst phase, and where the SN was initially faint and then
rapidly brightened due to interaction with circumstellar material (abridged).
