Blood clots can cause a variety of different problems, some as minor to little pinpricks of pain to highly lethal conditions, such as Heart attacks.
Blood clots are split into two different categories; Thrombi (or stationary clots) or Embolism (traveling clots, Thrombi that have broken and begin flowing through the blood stream. Essentially blood clots cause problem by obstructing the flow of blood, which is vital to get oxygen and nutrient to cells, to bodily tissues proximal to the clot. Depending on the organ and where its at lethal conditions can arise; Myocardial Infractions (heart attacks, heart cells begin dying), CVA/TIA (cerebrovascular accidents/ Transient Ischemic Attacks, or strokes... Obstructions of the brain), or pulmonary embolisms (clots that obstruct blood flow in the pulmonary vasculature).