  Using the $e^+e^-$ pair spectrometer at the 5 MV Van de Graaff accelerator at
the Institute for Nuclear Research, Hungarian Academy of Sciences (ATOMKI),
Krasznahorkay et al. have claimed a 6.8$\sigma$ excess at high $e^+e^-$ opening
angles in the internal pair creation isoscalar transition
$^8\mathrm{Be}(18.15)\to\,^8\mathrm{Be}\,e^+e^-$. A hypothetical gauge boson
with the mass circa $17$ MeV, "X17", has been proposed as an explanation for
the excess. We show that the observed experimental structure can be reproduced
within the Standard Model by adding the full set of second-order corrections
and the interference terms to the Born-level decay amplitudes considered by
Krasznahorkay et al. We implement a detailed model of the ATOMKI detector, and
also show how experimental selection and acceptance bias exacerbate the
apparent difference between the experimental data and the Born-level
prediction.
