RSS feed has errors at math.SE

Question

I have been using a feed for a specific tag ([set-theory]) of the Mathematics site on my WP blog. Right now it returns the following error message:

RSS Error: This XML document is invalid, likely due to invalid characters. XML error: not well-formed (invalid token) at line 805, column 34

I looked at the text dumped at the RSS link and the offending line has a "^L" character (that's the way it shows in Emacs) right before the last "filters" word. It looks like this:

<ol>
<li>The union of a chain of ^Lfilters is a ^Lfilter.
^O2. A maximal ^Lfilter is an ultra^Lfilter. 
How I can use Zorn's lemma to find the maximal filter?</li>
</ol>

The complete entry looks like this as plain text:

<entry>
    <id>https://math.stackexchange.com/q/1472767</id>
    <re:rank scheme="http://math.stackexchange.com">1</re:rank>
    <title type="text">Show that every proper filter on a set X can be extended to a proper prime filter?</title>
        <category scheme="https://math.stackexchange.com/tags" term="set-theory" />
        <category scheme="https://math.stackexchange.com/tags" term="filters" />
    <author>
        <name>Almoh</name>
        <uri>https://math.stackexchange.com/users/278782</uri>
    </author>
    <link rel="alternate" href="https://math.stackexchange.com/questions/1472767/show-that-every-proper-filter-on-a-set-x-can-be-extended-to-a-proper-prime-filte" />
    <published>2015-10-10T03:31:20Z</published>
    <updated>2015-10-10T05:15:44Z</updated>
    <summary type="html">


        &lt;p&gt;Are the following enough to complete the proof &lt;/p&gt;

&lt;ol&gt;
&lt;li&gt;The union of a chain of filters is a filter.
2. A maximal filter is an ultrafilter. 
How I can use Zorn&#39;s lemma to find the maximal filter?&lt;/li&gt;
 &lt;/ol&gt;

    </summary>
</entry>

I assume that when this entry is deep enough it won't bother any more, but anyway there was a problem

Answer 1

I did some digging after my comments and can confirm that there is something funky going on with that post. Firefox, which has generally been pretty good to me as far as Unicode support goes shows me this when editing the question:

As you can see there are several characters that aren't parsing correctly. I'm assuming that they are - characters, or other punctuation that got corrupted somewhere between typing the question in, and the question hitting the database. When I grab the question feed for that question, my XML parser gives up.

I suggested an edit to the original post that removes the invalid characters, that has been approved. Now that the characters are no longer in the generated RSS XML data, the feed parses correctly.

I'll leave it to a developer to make the call about whether the behavior is a fluke or whether there is a sanitization bug somewhere between question submission and feed generation. So at least until another user with the same keyboard layout asks a question the feeds should be safe. :)