Engineering education often makes reference to historical laws dating back to a time when sci ence was still at an early stage of development. An exam ple is the principle of displacement and moderation of che mical equilibrium formulated by Le Chatelier in various f orms between 1884 and 1933. The principle was never given a proper mathematical form, and later workers restated it in increasingly ambiguous verbal terms, subject to doubtful or plainly wrong interpretations. We show by an examination of Le Chateliers works that in fact he dismissed as incorrect the version usually associat ed nowadays with his name. His definitive formulation was much more specific, referring explicitly to changes in temperature, pressure and composition of a system. We compare this version with the predictions obtained from rigorous thermodynamic considerations. The results show that Le Chateliers approved principle is also incorrect as a whole, but can be accepted as two se parate statements, one for temperature and pressure changes in isolated systems, and the other for mass perturbati ons in isothermal, isobaric ideal mixtures. Finally, we re flect on the disadvantages of using unsound or loose versi ons of this principle in engineering education instead of a formal treatment.