Some, but not all, of the mistakes a person makes when acting in apparently necessary self-defense are reasonable: we take them not to violate the rights of the apparent aggressor. I argue that this is explained by duties grounded in agents' entitlements to a fair distribution of the risk of suffering unjust harm. I suggest that the content of these duties is filled in by a social signaling norm, and offer some moral constraints on the form such a norm can (...) take. (shrink)