A serum grouping result for ABO conducting tests shows hemolysis. Why?
It simply means red cell incompatibility. There are various different blood groups with specific antigens. Hemolysis would miserable cell destruction due to an existing antigen and antibody reaction beside incompatible blood types.
Check out this wiki page for details:
"O" group people moderately often own hemolysins - their anti-a and anti-b will hemolyse the test cell. That is why ABO incompatibilities of a fetus with an "O" mother can be fundamentally dangerous.