Suppose one person posts a partial answer to a question. (Perhaps the question had many parts and the answerer didn't answer every part.) And that answer is not CW. May another user edit that answer so as to supply what (s)he thinks is the missing info?
(I've just been sent a case of this. I thought "No, (s)he may not" but I am not sure of the etiquette, so I decided to skip the suggested edit -- someone more knowledgeable about the etiquette can deal with it. For the benefit of people reading these words long after I wrote them, my rep here reached 2000 only 5 days ago.)