  We present the results from multi-wavelength observations of a transient
discovered during the follow-up of S191213g, a gravitational wave (GW) event
reported by the LIGO-Virgo Collaboration as a possible binary neutron star
merger in a low latency search. This search yielded SN2019wxt, a young
transient in a galaxy whose sky position (in the 80\% GW contour) and distance
($\sim$150\,Mpc) were plausibly compatible with the localisation uncertainty of
the GW event. Initially, the transient's tightly constrained age, its
relatively faint peak magnitude ($M_i \sim -16.7$\,mag) and the $r-$band
decline rate of $\sim 1$\,mag per 5\,days appeared suggestive of a compact
binary merger. However, SN2019wxt spectroscopically resembled a type Ib
supernova, and analysis of the optical-near-infrared evolution rapidly led to
the conclusion that while it could not be associated with S191213g, it
nevertheless represented an extreme outcome of stellar evolution. By modelling
the light curve, we estimated an ejecta mass of $\sim 0.1\,M_\odot$, with
$^{56}$Ni comprising $\sim 20\%$ of this. We were broadly able to reproduce its
spectral evolution with a composition dominated by helium and oxygen, with
trace amounts of calcium. We considered various progenitors that could give
rise to the observed properties of SN2019wxt, and concluded that an
ultra-stripped origin in a binary system is the most likely explanation.
Disentangling electromagnetic counterparts to GW events from transients such as
SN2019wxt is challenging: in a bid to characterise the level of contamination,
we estimated the rate of events with properties comparable to those of
SN2019wxt and found that $\sim 1$ such event per week can occur within the
typical GW localisation area of O4 alerts out to a luminosity distance of
500\,Mpc, beyond which it would become fainter than the typical depth of
current electromagnetic follow-up campaigns.
