  Neutrino oscillations as suggested by the recent Super-Kamiokande result
imply that lepton numbers could be violated, and Z$\to \mu^{\mp} +\tau^{\pm}$
is a typical manifestation. We point out that for this mode, the GIM
cancelation is much milder with only a logarithmic behavior $\log (m_3 /m_2)$
where $m_j$ is the neutrino mass. The ratio $\Gamma$(Z $\to \mu^{\mp} +
\tau^{\pm})/\Gamma($Z $ \to \mu^{-} + \mu^{+})$ could be about
$10^{-5}-10^{-6}$, in sharp contrast with the vanishingly small rate
$\tau^{\pm}\to \mu^{\pm} + \gamma$ strongly suppressed by a quadratic power
$(m_3^2-m_2^2)/ M_{\rm W}^2$. Being complementary to neutrino oscillation
experiments, measurements of Z$\to \mu^{\mp} +\tau^{\pm}$ -- which are at hand
with the present colliders -- would give one more constraint to the lepton
mixing angle $\sin 2\theta_{jk}$ and the neutrino mass ratio $m_j/m_k$.
