This question already has an answer here:
- Duplicate banner breaks LaTeX 2 answers
To demonstrate the problem, I voted to close this question as a duplicate of https://math.meta.stackexchange.com/questions/12237/show-that-an-element-has-order-p-in-s-n-iff-its-cycle-decomposition-is-a-pro. Notice the $S_n$ in the question title. This is written in the source text as
S_n, which normally renders as a capital letter S with a subscript letter n, thus: $S_n$.
In the previous paragraph, I mentioned only the URL of the question and the software renders the title in place of the URL, hotlinked to the original question. The rendered version of the title properly renders the $S_n$.
Voting to close a question as an exact duplicate appends a comment to the question; I voted to close this very question as a duplicate in order to generate such a comment, which you can see below. The comment text includes a hotlink to the earlier question that the new one is claimed to be a duplicate of.
The bug is that the $S_n$ in the comment is rendered incorrectly, as $S\_n$ instead of as $S_n$.
That $S\_n$ appears in the source text of the comment as
S\_n instead of as
S_n. The backslash is spurious, and should not be there.