view-source:http://parsoid-lb.eqiad.wikimedia.org/mediawikiwiki/Thread%3AExtension_talk%3ALinkedWiki%2FNotice%3A_Undefined_index%3A_Beschrijving_in_%2Fvar%2Fwww%2Fwikifarm-mw1.19%2Fextensions%2FLinkedWiki%2FLinkedWiki.php_on_line_283%2Freply_%282%29?oldid=777881
includes some control characters that are [not allowed by HTML 5](http://www.w3.org/TR/html-markup/syntax.html#syntax-text). Note it's also [not allowed](http://www.w3.org/TR/html5/syntax.html#character-references) as an entity.
That post has 0x8, a non-space (as defined by HTML 5) control character.
This caused one of the issues with {T92303}. We'll probably work around it on our end, but it would be good for the parsing team to look at this issue.
UPDATE 2020-04-10: both the legacy wikitext parser and Parsoid allow these characters into HTML, and they cause pedantic errors but are allowed into the DOM; see discussion below.