I can replicate this but it looks like a gadget might be doing this as I can't replicate this for anonymous users in safe mode: https://en.wikipedia.org/wiki/Svchost.exe?safemode=1

@Izno any ideas if there are any default on gadgets that might be causing this to happen?

There are only 3 gadgets on by default in the en wiki main space without other restrictions: defaultsummaries, switcher, and referenceTooltips. I doubt any of those have changed recently much less to cause an issue like this one. And none of those have the same title as the ones on he.wiki (which could use some adjustments to when their default gadgets are loading).

This also doesn't happen for me logged out on Firefox for Android mobile (what I'm looking at right now).

So, pretty sure this isn't a local wiki issue. Does the user have any extensions running for Chrome? I'm somewhat intrigued this happens in one case on he wiki and not the others -- potentially implicates the BIDI algorithm. Also intriguing it's just this button.

I can replicate the issue in https://en.wikipedia.org/wiki/Svchost.exe in an incognito window.

I'll see if I can debug it further.

When outside safe mode this mysterious HTML appears inside the header element. What's weird is that is VERY old HTML (e.g. over 2 months). I cleared the cache via action=purge and the issue fixed itself. https://en.wikipedia.org/wiki/Svchost.exe is now fixed.

This makes me wonder if Varnish is not working on titles with "." in the name. I'm not sure what the appropriate tag is on Phabricator for varnish issues?

<label id="vector-toc-collapsed-button" class="cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet vector-button-flush-left cdx-button--icon-only" for="vector-toc-collapsed-checkbox" role="button" aria-controls="vector-toc" tabindex="0" title="Table of Contents">
						<span class="vector-icon mw-ui-icon-wikimedia-listBullet"></span>
						<span>Toggle the table of contents</span>
					</label>

Issue is still occuring on https://en.wikipedia.org/wiki/.example
Here is the HTML at time of writing: https://pastebin.com/GTtXZZFK

We had another report of the same bug on T349988. Purging the page fixed it. But that title did not have a "." in the title (https://en.wikipedia.org/wiki/Infinity)