  We consider the stable compactification $\bar {\mathfrak H}$ of the moduli
space of Horikawa surfaces with $K_X^2 = 2p_g(X) -4$.
  When $K_X^2 =8\ell$ we show that the closures of the two components
$\mathfrak H^{\mathrm I}$ and $\mathfrak H^{\mathrm {II}}$ of the Gieseker
moduli space intersect, for $\ell>2$ in a divisor parametrising explicitly
described semi-smooth surfaces.
  With growing $K_X^2$ we find an increasing number of generically non-reduced
irreducible components in the same connected component of the moduli space of
stable surfaces.
