A player has all the cards to be visited by one noble, but at the end of their turn they forgot about the noble tile. If another player in their turn also have all the cards for this noble who gets the noble tile?
Taking a noble is not optional, so the player who actually earned it first should get the noble as soon as it's discovered that he should have gotten it. By continuing to the next player without giving the noble to the person who earned it, an illegal mistake has been made, and if an illegal mistake can be easily fixed, then it should be.
It is impossible to refuse the visit from a noble, which is not considered to be an action.