Policy-makers frequently face substantial uncertainties and are required to cope with alternative scenarios that depict possible future developments. This paper argues that scenario reasoning is prone to suffer from characteristic mistakes. Probabilistic fallacies quantify uncertainties in an illegitimate way. Possibilistic fallacies systematically underestimate the full range of uncertainty, neglect relevant possibilities or attempt to represent a space of possibilities in an oversimplified way. Decision-theoretic fallacies, finally, fail to take the full range of uncertainties into account when justifying decisions, or misinterpret possibility statements by assigning them a special decision-theoretic meaning.