Fixed in the next build (and retroactively so, since those diffs aren't cached).
Getting the diff for the activity list accidentally Markdown-converted the post text twice. That's usually not an issue, since the Markdown conversion is mostly idempotent (and that's why, despite being broken for one and a half years, this has only been noticed now). When syntax highlighting hints come into play however, this idempotence is gone.
The first conversion adds the
class="lang-foo" prettify information. The second conversion sees a
class attribute on a
<pre> tag, which the HTML sanitizer doesn't allow. And unlike in the first conversion, where we know that the
class attribute was created by the conversion and thus is exempt, in the second conversion it looks like user-supplied, disallowed HTML, and hence the
<pre> tag is removed by the sanitizer.