On en.wikipedia.org, the browser connection security error page that has just been activated may contain invalid XML. It causes a parse failure when read using XmlDocument.Load() in the continuation to a HttpClient.GetStreamAsync() specifying an API Query request.
The 'img' start tag on line 21 position 38 does not match the end tag of 'a'. Line 22, position 3.
Although I honestly can't see what is wrong with the HTML. The attachment is snapped from a Fiddler trace.
I'm using HTTPS but I suspect I'm not properly enabling TLS 1.2. My workaround is to fix that so I don't see the error.