In this paper I will discuss the extent to which Umberto Eco’s Semiotics maintains the unstable and oscillatory equilibrium between conflicting matrices that is proper to humorous thinking and ambiguous figures such as the famous duck-rabbit illusion and Penrose’s impossible trident. To do so I shall summon the concept of bisociation (Koestler 1964): though never an item of Eco’s own philosophical toolbox, bisociation plays an important role in the creation of some of Eco’s most innovative theoretical contributions, insofar as they result from a possible/impossible graft between Structuralism and Pragmatism. For each, the question will be whether the implant produces self-consistent theoretical amalgams, or whether some degree of contradiction inevitably remains to generate the abovementioned duck-rabbit effect. I shall then analyse the role that such residual incongruities play in a theory that thrives on the self-voiding logic of humorous short-circuits.