Emergency department crowding is linked to medication errors and other preventable harm. Crowding requires providers to evaluate patients quickly under suboptimal conditions, such as in hallways or waiting rooms with inadequate nursing support, which may lead to diagnostic errors. This cross-sectional survey of emergency medicine physicians assessed how evaluating patients in the hallway or with a companion present changed their usual diagnostic practices. Researchers found that 90% of physicians altered their history-taking or physical examination, and 40% reported a diagnostic error or delay as a result. The most common missed diagnoses were suicidal ideation, abuse or neglect, and genitourinary system disease. A PSNet Perspective and a WebM&M commentary discussed strategies to reduce diagnostic errors in emergency departments.