  We discuss the issue of duality violations in hadronic tau decay. After
introducing a physically motivated ansatz for duality violations, we estimate
their possible size by fitting this ansatz to the tau experimental data
provided by the ALEPH collaboration. Our conclusion is that these data do not
exclude significant duality violations in tau decay. This may imply an
additional systematic error in the value of alpha_s(m_tau), extracted from tau
decay, as large as \delta alpha_s(m_tau) \sim 0.003-0.010 .
