We propose a dynamic model of neighbourhood watch schemes. While the state chooses punishment levels, apprehension of criminals depends on the watchfulness of citizens. We show that, contrary to standard intuition, crime levels can increase in punishments. This is because neighbourhood watch schemes can fall victim to their own success if recruitment of new members is driven by fear of crime - a finding that is in line with the empirical literature. We discuss the policy implications of this result and show how it extends to the more general problem of norm enforcement among interacting citizens.