Preparing novice physicians for an unknown clinical future in healthcare is challenging. This is especially true for emergency departments (EDs) where the framework of adaptive expertise has gained traction. When medical graduates start residency in the ED, they must be supported in becoming adaptive experts. However, little is known about how residents can be supported in developing this adaptive expertise. This was a cognitive ethnographic study conducted at two Danish EDs. The data comprised 80 h of observations of 27 residents treating 32 geriatric patients. The purpose of this cognitive ethnographic study was to describe contextual factors that mediate how residents engage in adaptive practices when treating geriatric patients in the ED. Results showed that all residents fluidly engaged in both adaptive and routine practices, but they were challenged when engaging in adaptive practices in the face of uncertainty. Uncertainty was often observed when residents' workflows were disrupted. Furthermore, results highlighted how residents construed professional identity and how this affected their ability to shift between routine and adaptive practices. Residents reported that they thought that they were expected to perform on par with their more experienced physician colleagues. This negatively impacted their ability to tolerate uncertainty and hindered the performance of adaptive practices. Thus, aligning clinical uncertainty with the premises of clinical work, is imperative for residents to develop adaptive expertise.