Here is one example:
Seventeen weak subgroup discrepancies were found in 86,687 donors units tested, making an overall incidence of 1:5,100 donors or 0.02%. Correct blood typing and labeling of an individual as donor and as a recipient is essential to prevent ABO incompatibility in case of weaker subgroups of A and B.
More here:
Things get more and more complicated … More 0- need to donate . And blood group testing needs to improve .