The wikitext :foo (space colon f o o ) is rendered by the PHP parser as <pre>:foo</pre> (as you would expect), but is rendered by Parsoid as a paragraph starting with a (span-wrapped) space:

<p data-parsoid='{"dsr":[0,5,0,0]}'><span typeof="mw:DisplaySpace mw:Placeholder" data-parsoid='{"src":" ","isDisplayHack":true,"dsr":[0,1,0,0]}'> </span>:foo</p>

This doesn't happen with any other special-seeming characters that I could think of, only colons. Things like ;foo (space semicolon f o o) and *foo are correctly rendered as <pre>s.

I discovered this because, while working on T209120, I found an old Flow post that contains <pre>:-(</pre>, which does not round-trip html2html cleanly:

$ echo '<pre>:-(</pre>' | ./parse.js --html2html --body_only
<p data-parsoid='{"dsr":[0,4,0,0]}'><span typeof="mw:DisplaySpace mw:Placeholder" data-parsoid='{"src":" ","isDisplayHack":true,"dsr":[0,1,0,0]}'> </span>:-(</p>

For a demonstration of this, see https://www.mediawiki.org/wiki/User:Roan_Kattouw_(WMF)/space_colon . The second line displays as a <pre> in the MW rendering, but changes to a paragraph when you open VisualEditor.