Designers' intentions are important for determining an artifact's proper function (i.e., its perceived real function). However, there are disagreements regarding why. In one view, people reason causally about artifacts' functional outcomes, and designers' intended functions become important to the extent that they allow inferring outcomes. In another view, people use knowledge of designers' intentions to determine proper functions, but this is unrelated to causal reasoning, having perhaps to do with intentional or social forms of reasoning (e.g., authority). Regarding these latter social factors, researchers have proposed that designers' intentions operate through a mechanism akin to social conventions, and that therefore both are determinants of proper function. In the current work, participants learned about an object's creation, about social conventions for its use and about a specific episode where the artifact was used. The function implemented by the user could be aligned with the designer's intended function, the social convention, both, or neither (i.e., an opportunistic use). Importantly, the use episode always resulted in an accident. Data show that the accident negatively affected proper function judgments and perceived efficiency for conventional and opportunistic functions, but not for designers' intended functions. This is inconsistent with the view that designers' intentions are conceptualized as causes of functional outcomes and with the idea that designers' intentions and social conventions operate through a common mechanism.