I recently came across this answer.
It states an untrue statement in which, I think, was in a joking manor.
What should I do when I see an answer like this?
The reasons for flagging an answer:
A wrong answer is not a reason to flag an answer. Wrong answers should not be flagged just because they are wrong. Leave a comment asking for references or providing a reputable reference that disproves the answer and downvote as appropriate.
Note that I deleted the provided answer since it wasn't an answer to the question. It was an attempt to be funny, and pretty much failed. If it was a comment, I would have left it alone, but I didn't feel that it added any value, so rather than migrating it to a comment, I just removed it entirely.
Your choice are (as ever)
If the answer has a negative score and you have more than 20K reputation then you can vote to delete.
What you shouldn't do is flag for moderator attention unless it clearly doesn't answer the question.
In this case it could be an answer (albeit a wrong one) - so there is always going to be a judgement call as to which side of the line it lies.