I think you'll find the position is that, at least three people have to independently challenge a question before the authority will review it (maybe Alex can verify this).
If the review process determines that the appeal is justified as the question is inappropriate the question is canned and all persons sitting that exam receive the mark(s) for the question.
There is always the risk that the reviewers may find that an incorrect answer had been inadvertently allocated to the question, e.g. the correct answer is 'X' but 'Y' has been incorrectly allocated as correct within the computer system. Anyone who selected 'Y' would lose the marks when 'X' was reallocated as the correct answer.