Many rebel groups ‘govern’, becoming increasingly institutionalised, accountable, and predictable. This is now well-accepted; however, less attention has been paid to another common observation: some rebel orders—and rebel-incumbent regimes—are more aptly characterised as unpredictable. We find that this is because they adopt vague mandates and delegate provisional authority. Our analysis shows that, in some cases, this (1) allows rebels to accommodate potentially incongruous pre-existing authorities and institutions, which can integrate unpredictability into early governance arrangements; (2) helps rebels cultivate social control from a comparatively weak position; and (3) has enduring legacies for rebel-incumbent rule. We illustrate this argument with Uganda’s National Resistance Movement (NRM) and explore broader relevance with the Afghan Taliban.