I have understood that if a question has community wiki (CW) status, then all answers must have it, too. I came across an exception in a question about refereeing. The answers by Chris Woodward and Joseph O'Rourke are non-CW, but all other answers to this CW question are CW. Is this supposed to be possible? Under what circumstances is this known or supposed to happen? It looks like a bug to me.
I have a guess why this happened: Joseph posted his answer at Aug 24 '10 at 23:25. He started editing his answer before the question was turned CW at 23:31, and the edit to the answer was saved at 23:44. The edited answer was saved as non-CW because the answer was non-CW when editing started, and this overrode the CW status inherited from the question. For Chris the time order is the same, so the same mechanism is possible. Can someone confirm or refute this guess? (Edit: This turns out to be false. It has nothing to do with edits. See the answer below.)
I feel I should add that I'm not upset by anyone receiving points for an answer that I think should be CW. I'm just curious.