Methods, systems, and machine readable media storing machine-readable instructions for enhanced classification of marginal instances are described. In one aspect, a classification system includes a cascade of forward classification stages and a marginal instance classifier. Each of the forward classification stages is configured to classify an instance into one of a rejected class, an unconditionally accepted class, and a conditionally accepted class. The cascade is configured to reject each instance classified into the rejected class by at least one of the forward classification stages, accept each instance classified into the unconditionally accepted class by all of the forward classification stages, and conditionally accept each instance classified into the conditionally accepted class by at least one of the forward classification stages. The marginal instance classifier is configured to classify each instance conditionally accepted by the cascade into one of a rejected class and an accepted class.