The 2011 duty hour regulations for resident physicians were controversial as soon as they were announced, and initial survey data revealed considerable dissatisfaction among both residents and residency program directors. This longitudinal cohort study surveyed interns working before and after the 2011 regulations and found that, although interns worked slightly less after the new regulations were implemented, they did not sleep more and were no less likely to report symptoms of depression. Most concerningly, self-reported medical errors increased significantly. The study corroborates prior studies conducted after the earlier (2003) duty hour regulations and illustrates how unintended consequences can result when rigid rules are applied within a complex adaptive system.