Pulmonary embolism
The worst kind of pulmonary embolism is caused by a saddle embolus.  This kind of embolus blocks the pulmonary artery, which carries blood from the right side of the heart to the lungs.  This makes it impossible for any blood to get through to the lungs.  Since no blood can get out to the rest of the body, the person's blood pressure drops and they can go into shock.  A saddle embolus is a very serious medical emergency.  Many people with this type of embolus die.