Is there variation across cultures in what counts as a lie? Here we present evidence for a potentially unique conceptualization of lying in Shuar-Achuar communities in Ecuador, contrasting this conceptualization with people in twelve other countries and non-Shuar-Achuar Ecuadorians. In Shuar-Achuar communities, but not others, predictions of the future that turn out to be false are considered lies, even when the events that render them false are unforeseen. Failed commitments, on the other hand, are not seen as lies when unforeseen events prevent them from being kept. To explain this phenomenon, we suggest that there is an epistemic norm that regulates predictive speech acts in Shuar-Achuar communities, linked to the view that the future can be known under certain special circumstances. This norm holds that claiming knowledge of the future is a form of lying when events prove the prediction false. Commitments, on the other hand, do not imply certainty about the future and so are not considered lies when circumstances prevent them from being fulfilled. In addition, we found several other factors that influence whether speech acts are categorized as lies, including the speaker's expertise, group membership, and the nature of the outcome.