Noncontingent reinforcement (NCR) has emerged as a treatment package for severe behavior problems. Although concerns about potential side effects (such as incidental reinforcement) have been raised, there have been few reported negative side effects in published studies to date. In this article, we report an NCR treatment evaluation for severe aggression that produced (a) an extinction burst and (b) incidental reinforcement. These side effects were evaluated by examining within-session response patterns and response distributions. As a solution, a brief omission contingency was added to the reinforcement schedule. The omission contingency resulted in decreased aggression rates.