Competition policy is a subject of often heated debate. Competition authorities, seeking to prevent or battle anticompetitive acts in complex cases to the best of their abilities, regularly find themselves advised by rival economic theories and disputed empirical analyses. As a consequence, there is a real possibility that they may occasionally err, missing true violations of competition law or finding firms liable that actually had no other intentions than good competition. In this paper, possible consequences of such imperfect competition law enforcement on firm strategies are considered. In a simple model of collusion, it is found that the incidence of anti-competitive behavior increases in both types of enforcement errors: Type II errors decrease expected fines, while Type I errors encourage industries to collude precautionary when they face the risk of a false conviction. Hence, fallible antitrust enforcement may stifle genuine competition, thus stimulating the very behavior competition policy is meant to deter. When enforcement errors are non-negligible, competition authorities run the risk of being over-zealous, in the sense that welfare is best served by an authority that is selective and conscientious in its targeting of alleged anticompetitive acts.