  We study Waring rank decompositions for cubic forms of rank $n+2$ in $n+1$
variables. In this setting, we prove that if a concise form has more than one
non-redundant decomposition of length $n+2$, then all such decompositions share
at least $n-3$ elements, and the remaining elements lie in a special
configuration. Following this result, we give a detailed description of the
$(n+2)$-th Terracini locus of the third Veronese embedding of $n$-dimensional
projective space.
