What happens if a hepatitis b positive person (who is not aware their status) gets vaccinated for hepatitis b?! nothing? or adverse reaction?

None: If hepatitis b positive person (positive hepatitis b surface antigen) receives vaccine for hepatitis b, it will not be effective. So, nothing will happen, except for vaccine related adverse reaction, if any.

Nothing: A hepatitis b positive person already has been exposed to the wild virus. The immune system already has responded and has either eliminated the hepatitis v virus or it's gone into a chronic infection status. Presenting the Hepatitis B Vaccine to the immune system is redundant - it already has responded. You might notice soreness or redness at the injection site.