An unpredictable ghost peak was intermittently observed during the impurity separation of cefaclor and formulation by high performance liquid chromatography (HPLC) with a content from below the reported threshold to approximately 0.3% in different laboratories. Through a series of investigations, the ghost peak was identified as an unusual on-column degradant of cefaclor formed under elevated column temperature but was not an actual sample impurity. The chemical structure of the degradant was determined by spectroscopic methods, including high resolution mass spectrometry (HRMS) and H-NMR. Consequently, the unknown peak was identified as a C-4 oxidative decarboxylation analog of cefaclor. The formation mechanism of the analog is proposed, and it is suggested that elevated column temperature during HPLC analysis has a profound effect on the degradation. Dissolved oxygen in the mobile phase may promote the formation of the ghost peak. The degradation can be suppressed by using a column temperature below 30 °C. Moreover, several other prevention measures are suggested based upon the results of the investigation.