I noticed that, after I replied, the interface wasn't updated to show that I have become subscribed to the topic (although I was, and refreshing the page shows it correctly). The next time it happened I looked in the browser console, and found:
jQuery.Deferred exception: element is undefined getSubscribedStateFromElement@https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core&skin=vector-2022&version=k3kzd:35:805 updateSubscriptionStates@https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core&skin=vector-2022&version=k3kzd:42:635 updateAutoSubscriptionStates@https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core&skin=vector-2022&version=k3kzd:44:742 controller.js/init/<@https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core&skin=vector-2022&version=k3kzd:15:674 mightThrow@https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core&skin=vector-2022&version=k3kzd:360:648 Deferred/then/resolve/</process<@https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core&skin=vector-2022&version=k3kzd:361:309 undefined jquery.js:3783:17