PEBKAC related really. Trying to submit a post, not using the new “QuickPress”, I was getting an error about the function enqueue_comment_hotkeys_js not being defined. A quick google showed one other victim, who also suffered, but simply commented out the call. I took a little extra time, and checked out some of the other google results, which were all just source code views, which told me it did exist.
Turns out, the issue was because of my FTP client saw the date on wp-admin/includes/comment.php was older in the zip file than the one on my server, so didn’t update it. Why it was older in the zip, I’ve no idea, so I redeployed the zip file again, and told File!Zilla to overwrite all files.