To reproduce, go to https://en.wikipedia.org/w/api.php?action=query&titles=%3C&format=jsonfm.
Everything after the < is colored blue, and the HTML is obviously invalid:
[...]
<pre style='white-space: pre-wrap;'>
{
"query": { "pages": { "-1": { "title": "<span style="color:blue;"><", "invalid": "" } } }
}
</pre>
[...]
Version: 1.24rc
Severity: minor
URL: https://en.wikipedia.org/w/api.php?action=query&titles=%3C&format=jsonfm