Pré-publication, Document de travail: We use a non-Bayesian approach to uncertainty which allows for both optimism and pessimism in a simple global game, where each signal can exhibit a bias which is ambiguous. We underline a symmetry between two models of financial crises: a liquidity crisis model, and a currency crisis model. We show that one model with pessimism becomes similar to the other model with optimism, and vice versa, which leads ambiguity to have opposite effects in the two models. We can also rationalize non-neutral effects of shifts in "market sentiment" in these models.