When experts find that a question was wrong, ambiguous or outside the syllabus, the recruiting body drops it in the final answer key. What happens to the marks is decided by a rule published in advance.
The two common rules
Either every candidate is given full marks for the dropped question, or the question is removed and the paper is evaluated out of fewer marks, sometimes scaled back to the original total.
Who benefits
Under the first rule, candidates who left the question blank or answered it wrongly gain the most. Under the second, no one gains marks, but everyone's percentage shifts slightly.
Changed answers
A question can also be kept with a different correct option, or with more than one option accepted. Your marks are then recalculated against the revised key.
Where to find the rule
It is stated in the notification or in the notice issued with the final key. Rules differ between recruiting bodies and even between exams of the same body.
What you can do
Nothing after the final key, which is why the objection window on the provisional key matters. Raise genuine objections then, with a standard source.
Provisional and final keys are tracked on the answer keys page.