The error on the linked one is happening on a diff of a talk page. However, it doesn't happen to me when I visit that page.

The specific error just boils down to "a comment referenced in the serialized thread doesn't actually exist on the page". (It's in ThreadItem.js on line 111.) It's few enough errors for 7 days that it probably requires some specific user-configuration to happen -- a userscript that meddles with the content on diffs would be enough to do it.

mediawiki.base.js:504 Uncaught TypeError: Cannot read properties of null (reading 'parentNode')
    at ThreadItem.js.ThreadItem.static.newFromJSON (ThreadItem.js:113:31)
    at infuse (ThreadItemSet.js:32:34)
    at ThreadItemSet.js:36:57
    at Array.map (<anonymous>)
    at infuse (ThreadItemSet.js:36:35)
    at ThreadItemSet.js:43:3
    at Array.forEach (<anonymous>)
    at ThreadItemSet.js.ThreadItemSet.static.newFromJSON (ThreadItemSet.js:42:10)
    at Object.init (controller.js:324:37)
    at reallyInit (dt.init.js:38:14)

I see this in the console on different pages from time to time. With safemode=1 it doesn't seem to happen. I was afraid Convenient-Discussions does this, but no. There seems to be some race condition there, and I don't always catch this (at times, another bug happens instead of this one, namely T367639), so you may need to try it in different ways to see the bug:

• Open Chrome
• Type https://www.mediawiki.org/wiki/Local_development_quickstart#Windows in the address bar, press Enter
• Before the page is loaded, go to another tab
• When the first tab is loaded, switch to it
• See the above error in the console

Could this be related to T348680?

Should be fixed by https://www.mediawiki.org/w/index.php?title=MediaWiki%3AGadget-tabbedwindow.js&diff=6588257&oldid=6583748

DT expects to find the HTML it generated when it infuses the page, so lets preserve the original headings (hidden) instead.

I can replicate on https://bn.m.wikivoyage.org/wiki/%E0%A6%89%E0%A6%87%E0%A6%95%E0%A6%BF%E0%A6%AD%E0%A7%8D%E0%A6%B0%E0%A6%AE%E0%A6%A3:%E0%A6%A8%E0%A6%BF%E0%A6%AC%E0%A6%A8%E0%A7%8D%E0%A6%A7_%E0%A6%AA%E0%A7%8D%E0%A6%B0%E0%A6%A4%E0%A6%BF%E0%A6%AF%E0%A7%8B%E0%A6%97%E0%A6%BF%E0%A6%A4%E0%A6%BE_%E0%A7%A8%E0%A7%A6%E0%A7%A8%E0%A7%AA/%E0%A6%A8%E0%A6%BF%E0%A6%AC%E0%A6%A8%E0%A7%8D%E0%A6%A7_%E0%A6%A4%E0%A6%BE%E0%A6%B2%E0%A6%BF%E0%A6%95%E0%A6%BE - I can look into the associated gadget later and suggest a fix.

Hmmm I can replicate this in safe mode so maybe this is not gadget related? https://bn.m.wikivoyage.org/wiki/%E0%A6%89%E0%A6%87%E0%A6%95%E0%A6%BF%E0%A6%AD%E0%A7%8D%E0%A6%B0%E0%A6%AE%E0%A6%A3:%E0%A6%A8%E0%A6%BF%E0%A6%AC%E0%A6%A8%E0%A7%8D%E0%A6%A7_%E0%A6%AA%E0%A7%8D%E0%A6%B0%E0%A6%A4%E0%A6%BF%E0%A6%AF%E0%A7%8B%E0%A6%97%E0%A6%BF%E0%A6%A4%E0%A6%BE_%E0%A7%A8%E0%A7%A6%E0%A7%A8%E0%A7%AA/%E0%A6%A8%E0%A6%BF%E0%A6%AC%E0%A6%A8%E0%A7%8D%E0%A6%A7_%E0%A6%A4%E0%A6%BE%E0%A6%B2%E0%A6%BF%E0%A6%95%E0%A6%BE?debug=1&safemode=1

It's something about the heading getting adjusted. The failure on that page is because document.getElementById('h-স্থান-20241004194000') isn't finding anything.

desktop
mobile

...so on mobile there, we've not got the data-mw-comment-start element for the headings.

One more case of this error that I could localize – this time because of RevisionSlider:

1. Go to https://en.wikipedia.org/w/index.php?diff=1262221555&oldid=1262192427&title=Project:Village_pump_(proposals)&safemode=1&dtenable=1 (note &safemode=1 in the URL – it's not some user script interfering).
2. Click "Browse history interactively".
3. Press "Previous edit" (but not "Next edit" for some reason).

In general, I would appreciate making the code more forgiving, because at this point I'm kind of used to scripts, including mine, changing something in the DOM for what DiscussionTools wasn't ready.