Do most transfusion-related human errors occur inside or outside the transfusion service and what are the implications?

Most transfusion-related human errors occur outside the transfusion service, which suggests that hospital-wide efforts at prevention are required.

For an example of a hospital-wide education program, see this paper:

Clark P, Rennie I, Rawlinson S. Quality improvement report: Effect of a formal education programme on safety of transfusions. Br Med J 2001 Nov 10;323(7321):1118-20.