If a question contains a mathematical error that could be pointed out in a posted answer, or is ill-formulated for reasons that could be explained in a posted answer is that a reason to close the question, and more importantly, is there a standard policy or convention on that?
There is a lot of variations here, each quite different. Let me go through a few with some examples (disclaimer: most, if not all, will be answers I gave simply because it is easier to remember the cases where this happened with me).
- The question contains an error because it is about the error (or potential error).
This may seem rhetorical, but could be an interpretation of the situation you are asking. Obviously (I think), a question should not be closed because of that. Here is an example.
- The question is ill-formulated, in the sense that the person asking is missing some subtlety to make the question completely meaningful, but not in the sense that it is wrong.
Since this case may seem a little abstract, here is an example. I believe this case is a little more complicated and can have a lot of variation. For instance, answering these kinds of questions can be inviting yourself to a discussion which goes nowhere. And these kinds of question can oftenly fall under "unclear what you are asking". If that is the case, I would expect that closing would not be a clear-cut decision. Some people are more lenient towards unclear questions, some less.
- The question is wrong (i.e., asking a false statement).
I could not find an example for this, but I think that someone asking "Prove that $(0,1)$ with the standard topology is compact" would be good. Consider that it is also contextualized, for the sake of that not entering the point here.
This may also fall under the "unclear what you are asking" case. I don't know if the person would be content with knowing that $(0,1)$ is not compact, or if the person wants an example of a simple compact set and for some reason thought that this example should be true, or if they meant to say $[0,1]$. I could put all those things together in an answer and still not touch what OP wanted. And it is not clear for other users what OP wanted. Again, there is a high degree of personal interpretation if this is low quality or not, so closing is highly personal and dependent on the specific instance.
EDIT: Found one perfect example.
- The question has no context and is wrong.
This is a case where I am very inclining to closure, but I admit this is personal. Risking myself to be putting words in another person's mouth (and also risking mentioning part of the fuss of the now-deleted question, but because I think that there is a good point there), when it was said that "the question has no context and is mathematically wrong" I believe that you read this as the question (has no context) and (is mathematically wrong), but it should be read as the question (has no context and is mathematically wrong). Saying that this justifies a closure does not mean that each reason, separately, justifies it. It means that the adjuction of those two characteristics makes it a low quality question. At least, that is what I gathered from that, and I agree that this is, in general, good reason to close a question.
Directly and solely because of that, no (in general). But the particular way that the error is embedded in a question may be related to other issues which justify closure.