Background: Tibial plateau fractures (TPFs) are sometimes overlooked in the emergency room (ER). Using a national register covering 18 years we aimed to find out why and to evaluate if use of a specific radiographic decision rule, Pittsburgh Knee Rules (PKRs), could have reduced the number of overlooked TPFs. Methods: Medical records for 137 patients, prospectively registered during 18 years by the Danish Patient Compensation Association (DPCA) (a national register), were studied. The inclusion criterion was a delayed diagnosis of a fracture in the knee following a trauma. Case records, legal assessments, and evaluations by specialist doctors were reviewed, and the consequences of the delayed diagnosis for outcome and treatment were registered. Results: Only 58 patients (42%) had been evaluated according to PKRs. In 53 patient cases, the fracture was not diagnosed on radiographs obtained at the first medical contact. However, in 84% of these cases, the fracture was visible or was suspected by retrospective evaluation. 50 out of 79 patients, for whom X-rays were not obtained, were candidates for radiographs according to PKRs, 17 cases lacked information to evaluate by PKRs and 12 cases were not candidates. In 53% of all cases, it was evaluated that the fracture position had worsened at the time of diagnosis. A significant disability compensation was granted in 36% of cases due to the delayed identification of fractures, totaling 841,000 EUR. Conclusions: The major reasons for overlooking TPFs were 1) difficulty in recognizing the fractures on X-rays and 2) that X-ray decision rules were not employed. Two thirds of the patients, for whom a radiograph had not been prescribed, would have had an X-ray, if the PKRs had been used. Overlooking TPFs significantly increased patient disability in one third of cases. We recommend that healthcare professionals in the ER use X-ray decision rules in addition to clinical examination to avoid overlooking TPFs. When standard radiographs are evaluated as normal in patients that are clinically suspect of a TPF, oblique X-rays, magnetic resonance imaging (MRI) or Computed Tomography (CT)-scan should be considered.