What happens if a blood transfusion doesn't match your blood type?

You would pretty much die.  There are particles in A blood that would attack and destroy B blood type cells.  This would happen extremely quickly-- so it would be easy to catch, but probably fatal if too much blood was transfused.  Luckily the chances of this happening are slim.

there will be a hemolytic transfusion reaction. the recipient's body will produce antibodies which will destroy blood cells which they don't recognize. these is dangerous coz the recipient could die

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The person who has type A will have anti-B antibodies in his blood which would bind to the B cells and create a cascade of effects from binding complement destroying the red cells, producing shock with DIC and inducing renal failure due to the toxic byproducts.

Monitoring of a transfusion is mandatory and vital signs are checked during a transfusion. Any signs consistent with a hemolytic reaction would cause an immediate cessation of the transfusion and an immediate transfusion reaction workup initiated by a blood sample collection to determine if a reaction occurred. The volume of the blood infused should be minimized as the more blood infused would create the most damage.

The rest is pretty much symptomatic supportive care including dialysis if renal failure ensues.

Depends.
1] if you are an A, you would be given type A [ same antibodies as A ] otr O [ no antibodies to A ]
2] if they gave you B, or AB; both of those have some antibodies to plain A, which would cause the cascading effect described by Lab Guy.