What are the ramifications if a blood type is mismatched for the first time?

Tranfusion reaction. Preferably, only blood of the same type should be transfused, otherwise a transfusion reaction may occur ( fever, hemolysis etc..);however, those who have blood type "ab" may received other blood when their own type is not available for transfusion ( type ab=universal recipient), and those of type "o" must receive only blood of their own type (type o= universal donor).