Background: Care transitions between general practice and hospital are hazardous regarding patient safety. For developing an improvement strategy adjusted to local settings, understanding of type and potential causes of transitional safety incidents (TSIs) is needed. Objectives: To provide a broad overview of the nature of TSIs reported by patients and healthcare professionals. Methods: We collected data (2011–2015) from three hospitals and 56 affiliated general practitioners (GPs) in two Dutch regions (one urban, one rural). We collected data from patients through a survey, interviews and incident reporting weeks, and from GPs and hospital specialists through incident reporting systems, surveys, interviews and focus group discussions. We classified reported TSIs according to type, cause and severity. Results: In total, 548 TSIs were reported by 411 patients and 137 healthcare professionals; 368 of 548 TSI reports contained sufficient information for classification into aspects of the care transition process, 191 of 548 for cause, and 149 of 548 for severity. Most TSIs concerned handover correspondence from hospital to GP (26%), referral (14%) and communication/collaboration (14%). Concerning cause, reported TSIs could be attributed to organizational (48%) and human factors (43%). Twenty-four percent concerned unsafe situations, 45% near misses and 31% adverse events. Patients and healthcare professionals reported differently on referral (17% vs 9%), repeated diagnostic testing (20% vs 1%), and uncertainty about assigned responsible physician (10% vs 3%). Conclusion: Reported TSIs typically concerned informational discontinuity. One third caused harm to the patient. Patients report different TSIs than healthcare professionals, suggesting a different view.