<p>As cardinal notes in his answer, the problem is caused by an extremely long MathJax expression that exceeds the normal width of the comment area.  Normally, MathJax does allow excessively long expressions to be broken over multiple lines, but that requires the expressions to actually contain some "natural" breakpoints such as commas or math operators.  If it's all just run-on text, it won't get wrapped.</p>

<p>Since the comments are formatted using HTML tables, the over-long math expression then causes the whole comments table to expand to accommodate it.  The obvious fix would be to set an explicit maximum width for the comment text, e.g. with:</p>



<pre><code>.comment-body {
    max-width: 630px;
    overflow: auto;
    overflow-y: hidden;
}
</code></pre>

<p>These CSS rules cause any over-wide comments to acquire a horizontal scroll bar instead of spilling over into the sidebar, like this:</p>

<p><img src="https://i.stack.imgur.com/k25Bg.png" alt="Screenshot"></p>

<p>Ps. While waiting for the SE devs to fix this styling issue properly, I've added the CSS fix described above to version 1.22 of the <a href="https://stackapps.com/questions/4486/stack-overflow-unofficial-patch">Stack Overflow Unofficial Patch</a>.  (Earlier versions, since v1.10, included a slightly different fix using <code>table-layout: fixed</code> instead.)  Please let me know if you encounter any unwanted side effects from it.</p>
