Care transitions that occur across healthcare system boundaries represent a unique challenge for maintaining high quality care and patient safety, as these systems are typically not aligned to perform the care transition process. We explored healthcare professionals' mental models of older adults' transitions between the emergency department (ED) and skilled nursing facility (SNF). We conducted a thematic analysis of interviews with ED and SNF healthcare professionals and identified three themes: 1) ED and SNF healthcare professionals had misaligned mental models regarding communication processes and tools used during care transitions, 2) ED and SNF healthcare professionals had misaligned mental models regarding healthcare system capability, and 3) Misalignments led to individual and organizational consequences. Overall, we found that SNF and ED healthcare professionals are part of the same process but have different perceptions of the process. Future work must take steps to redesign and realign these distinct work systems such that those involved conceptualize themselves as part of a joint process.