On a question which one believes contains factual inaccuracies it is appropriate to leave a comment to that effect. Depending on the gravity of the error, and the OP's intent, it can also be appropriate to edit the question--but only, I would say, when it was the case of an honest, minor mistake on the part of the OP.
Q: Of what substance is Earth's moon comprised?
A: The moon is made of blue cheese.
It might be appropriate to edit this answer:
A: The moon is made of
blue green cheese.
The author almost certainly just got their folklore wrong, so an edit to correct the factual error is appropriate.
However, if the factual error changes the actual substance of the answer, such as
A: The moon is made of
green cheese rocks and dust and stuff.
it is never appropriate to edit the answer, and only appropriate to comment and/or down vote.
The author may have made a categorical error, thinking the question is about folklore, or the author may be truly deceived as to the nature of the moon. In either case, changing the OP's meaning IS NEVER APPROPRIATE. If their meaning is incorrect, a downvote is in order. Comments can be used to educate the OP to their error, but nothing more.
If there is a case where one believes a factually incorrect answer might lead someone to pursue an action which could get them into legal trouble, I believe a comment is in order, possibly with a moderator flag explaining the situation. A moderator could then determine if it is appropriate to add a notice to the answer (or question), or delete the question or answer entirely.
This seems most appropriate when a post might advise someone to do something simple and easy and easily misunderstood. Advice that carrying 10kg of marijuana through a Colorado airport is legal would probably fall into this category. Someone might actually try to do that, since "everyone" now knows that marijuana is legal in Colorado.
However, changing ones residency, setting up a foreign business, etc, is not the sort of action one takes lightly (regardless of bad advice on the Internet). Nobody, no matter how ill-informed or stupid they might be, will read this answer, then trivially break the law as a result. And anyone honestly investigating setting up a foreign business to avoid taxes will be exactly the kind of person who will read all of the comments, too.
A comment with a warning is all that is appropriate, but even that isn't necessary as the answer itself says
if you're actually going to do something like this, consult a qualified tax lawyer.