Many harmless organisms gain a survival advantage by mimicking venomous species. This is the case of the endangered smooth snake (Coronella austriaca), which mimics venomous vipers. Although this may protect the smooth snake against most of its natural predators, it may render them at greater risk of mortality from humans, who are more inclined to kill species, such as vipers, that they consider dangerous. This may cause an evolutionary mismatch, whereby humans may counteract the natural advantage of mimicry. We explore this possibility evaluating the willingness of humans to kill smooth snakes versus the adder (Vipera berus), as well as their ability to discern them in the Åland Islands. Our results show that, even when respondents did not wish to kill the smooth snakes, they were often mistaken for adders, which they were willing to kill. Altogether, viper-mimicry brought about a 2.3-fold increase in the likelihood of smooth snakes being killed upon human encounter. These results open up the possibility that naturally selected mimicry can pose a threat to endangered snakes in human influenced habitats. We discuss the potential for this to be the case, and highlight the importance of protecting entire mimicry complexes, rather than single species, when the endangered species is a mimic.