Esther Ingliss-Arkell has an interesting short description of “What Happens When You Get The Wrong Blood Type?” on io9.
The first sign of a transfusion gone wrong is "a feeling of impending doom." This is a legitimate medical symptom, and doctors who regularly work with blood transfusions are told to look for it. Other sign of a mismatched blood type is the usual immune system warning flags — flu-like fever, ache, and chill, as well as a burning sensation at the injection site.
I teach the ABO system as part of my usual human genetics lectures, and I’ve never read a great description of the range of transfusion reactions—I was really only aware of the most catastrophic type. So this can be an interesting piece for those teaching ABO as well as those who’ve just always wondered.