It was supposed to be a routine hospital stay. Geraldine Oswald had developed an infection after breaking her shoulder last year, and she was hospitalized in November at Massachusetts General for treatment. But in a tragic mix-up, hospital staff gave the 76-year-old woman far too much of a blood thinning drug, making it impossible for her blood to clot when she began bleeding internally. Hospital officials later acknowledged the mistake and said it could have been prevented. According to her death certificate, Oswald hemorrhaged for 12 hours while in the hospital's care before she died. Family members said they plan to file a lawsuit in Suffolk Superior Court today naming five doctors, two nurses, and Massachusetts General Hospital in the wrongful death of Oswald, saying the Revere woman was supposed to be treated for a common infection, but instead, a nurse gave her a dose of the blood thinner, called Lepirudin, that was 30 times too high.