Answers being posted before closing invalidates the closing mechanism
Quite often, I see the issue of a stackizen posting their an answer while a question is being discussed/closevoted. This isn't always a problem, but it can cause issues when the question is off-topic, or lacking in details.
It's related to the FGitW problem, in that where I see this being an issue is when the answerer is quicker than the close-voters. I don't think this is wholly solved by reducing the number of close votes, my concern there being that we'd see too many false positives.
So where do I see this being an issue? There're three main stacks that I see this on, but I'm sure examples could be drawn from many more sites on our network.
Questions are often closed here when they lack details, or are recommendations (AKA Shopping Questions). The former question can be re-opened, but only once enough details are provided by the askers. When answers are posted before a question is closed, we get the awkward state of affairs where we have answers that need to be overhauled or deleted when more details come in. On the side of recommendation questions, if the querent gets their recommendation then the purpose of closing becomes invalidated.
Sci-Fi and Fantasy
Some questions on this site, need more details before being answerable. By and large these are story-identification questions. We also get a (now) smaller share of recommendation questions, which are not on-topic but have in the past been quickly answered. Sometimes it's not clear a question is generating recommendations until it's too late.
One unique use of this warning, on Puzzling specifically, is that askers wanting a puzzle solved need to provide a source. If they don't the question is supposed to be closed, to prevent people from using the site for exam help, for instance. But people get pretty excited about solving puzzles, and don't notice that the question looks like it needs to be sourced. It's quite a skill to spot, so you may only get 1-2 close votes initially before more notice the issue. Then you have the issue that the asker now has an answer, despite not being on topic. Any action after the fact is not effective.