There's a similar PHP 8.1 related deprecation notice in the Unit renderer on this line:

Perhaps another approach that could be considered, doing the colorizing in the browser using something like highlight.js (https://highlightjs.org/). This, of course, puts the CPU burden on the end user instead of the server.

That's correct, there are no magic words in my diff message. The template I'm given when I create a new diff contains text (I assume because of the Pirate language setting I've chosen) that causes Arcanist/Differential to choke when the message is committed, as "Spies" isn't a valid heading in the Diff message.

Ah, ok, I did a brief search to see if there was already a ticket, missed that one.

It also doesn't seem to happen on divs that have the code coverage gutter.

Chrome doesn't seem to do it, Firefox 29 (OSX).

Here's a screenshot of that same diff from T4682, but with this change in Arc: