Venous Thrombosis - blood clots which from in veins (as opposed to arteries) - are relatively common and potentially very dangerous (e.g. DVT).
Although it's firmly established that three factors (hypercoagulability, hemodynamic changes and endothelial injury - collectively known as Virchow's triad - make Venous Thrombosis more likely, the actual physiological process which triggers a clot to form in a vein is unknown.
“Epidemiological research throughout the last 50 years has provided the long list of risk factors for venous thrombosis that are known today. Although this has advanced our current understanding about the aetiology of thrombosis, it does not give us all the answers: many people have several of these risk factors but never develop thrombosis; others suffer from thrombosis but have none.”