Abstract: During the drilling operation of a semi-submersible drilling platform, after the pressure test on the subsea blowout preventer (BOP) stack was completed, a problem occurred where the test plug could not be pulled out smoothly. In the process of handling this issue, a serious jamming of the test plug by the rubber core of the annular blowout preventer happened, resulting in a long-term shutdown of the platform. This article reviews the occurrence and handling process of the incident, identifies the causes of the test plug's difficulty in being pulled out and its serious jamming by the annular blowout preventer through analyzing various phenomena and signs in the incident. Based on the cause analysis and the study of relevant materials, the paper puts forward preventive measures to avoid similar incidents, providing a reference for relevant practitioners.