I answered a question, missing part of the implementation (because I don't know about the OP's system and am waiting for a reply to my question asking his system). Four hours later, another user creates his own answer that only adds an implementation detail but is phrased as if it is an entire answer.
See question at How do aliases or redirects work?.
I make a comment pointing out that the other answer is not answering the question, to which the other person responds
The other steps to the process has been answered by other users. I'm adding a very important step to complete the process ...
However, if his answer was interpreted as an answer by the OP the OP would end up confused because if they implement the answer it won't come close to what they asked how to do. (They will end up redirected to a non-existent subdomain).
In this situation, I would add a comment to the original answer saying
you missed <x> or if I was in a reputaion-obsessed mood possibly answer with
You need to do everything <name-of-first-answerer> says, and then you need to do this and this
However, this other user has significantly more rep than me, so I think they may know something that I don't.
What is the proper action here for the other person? If the other person is in the wrong, what response should I give?