This question already has an answer here:
Over on Science Fiction & Fantasy, we've had a couple of answers containing some Unicode characters that spilled over onto the rest of the page.
The comments were deleted and edited to push the offending bit into history, as discussed on our meta.
So the direct problem has been solved.
However, should Unicode characters in a post be able to do this? Should the contents of an element be allowed to draw outside the boundaries of that element?
(I think they should not, hence this bug report).