The blood will coagulate and clog the vessels, which can lead to death within minutes or instant death, in addition, there will be a massive allergic reaction (anaphylactic shock). The severity of the effects depends on how much blood has already been transfused. In fact, there are more than 300 blood groups, if you take into account all systems and classifications. Now, according to the rules, a person can only be transfused with blood of the same group according to the AB0 system and rhesus.
According to the classics, in the blood there are two factors (antigens) - A and B, incompatible with each other. The first group - there are no factors, you can transfuse everyone, takes only his own. The second - there is only factor A, the third - only factor B, the fourth - there are both A and B. Blood of the fourth group is suitable only for the same, everything can take. Later, another factor was discovered - the Rh factor, which doubled variations. At present, 17 blood groups are determined in the USA. Accordingly, if a person is transfused with compatible blood of another group, everything will be fine, and if incompatible, he may die from aggregation (sticking together) of red blood cells.
As far as I know, a person will simply die after such an operation. It is possible that he will experience physical torment. I do not advise you to check either on yourself or on anyone else.