Le Chatelier’s principle is widely applied in undergraduate chemistry to predict responses to perturbations in equilibrium. Thus, it may come as a surprise to some that, if employed in too simplistic a fashion, Le Chatelier predictions can be inaccurate for certain constant pressure gas phase reactions. Here i present several thermodynamic analyses of this problem, along with a novel, simple mnemonic diagram that allows students to determine when simple Le Chatelier’s principle predictions are accurate when adding a gaseous reactant or product at constant pressure and temperature.