  The local coordination numbers of As$_2$Se$_3$ glass were determined by a
combination of anomalous x-ray scattering experiments, reverse Monte Carlo
calculations, and {\it ab initio} molecular dynamics simulations. The
well-known `8-$N$ bonding rule' proposed by Mott breaks down around the As
atoms, exceeding the rule by 7--26%. An experimental prediction based on
mean-field theory agrees with the present experimental and theoretical results.
The fourfold coordinated As atoms likely form As-As wrong bond chains rather
than ethan-like configurations, which is identified as the origin for the
breakdown of the `8-$N$ bonding rule'.
