  Covariant gauges lead to spurious, non-physical polarisation states of gauge
bosons. In QED, the use of the Feynman gauge, $\sum_{\lambda}
\epsilon_\mu^{(\lambda)}\epsilon_\nu^{(\lambda)\ast} = -\eta_{\mu\nu}$, is
justified by the Ward identity which ensures that the contributions of
non-physical polarisation states cancel in physical observables. In contrast,
the same replacement can be applied only to a single external gauge boson in
squared amplitudes of non-abelian gauge theories like QCD. In general, the use
of this replacement requires to include external Faddeev-Popov ghosts. We
present a pedagogical derivation of these ghost contributions applying the
optical theorem and the Cutkosky cutting rules. We find that the resulting
cross terms $A(c_1,\bar{c}_1;\ldots)A(\bar{c}_1,c_1;\ldots)^\ast$ between ghost
amplitudes cannot be transformed into $(-1)^{n/2}|A(c_1,\bar{c}_1;\ldots)|^2$
in the case of more than two ghosts. Thus the Feynman rule stated in the
literature holds only for two external ghosts, while it is in general
incorrect.
