It appears that an AFT-related JavaScript error is causing issues with the watchlist, as reported here on Village Pump: (see comments below)
JavaScript not loading on my Watchlist?
For some reason, JavaScript isn't loading on my Watchlist. This morning it worked fine, but in the past few hours JavaScript has not been loading on my Watchlist. For instance, I use "Enhanced recent changes", so diffs should be collapsed, but at the moment they are all expanded. I'm also using the Monobook skin. I see no errors appearing in my JavaScript Console in Firefox. JavaScript works fine on all other pages. At the moment my guess is that something was changed in the JavaScript for Watchlists, such as a new variable, that could be conflicting with one of my scripts, so no error is given but something is still broken. I emptied my JS and CSS pages, and still the same problem on my Watchlist, so something seems to be broken on Wikipedia's end. Gary King (talk · scripts) 18:48, 29 October 2012 (UTC)
Link Classifier wasn't working for me on my Watchlist after I read your post, but I just manually reloaded it and it then worked. I would suggest that, and then emptying your browser cache if it doesn't work.--JohnBlackburnewordsdeeds 18:56, 29 October 2012 (UTC)
I already did a hard refresh and cleared my cache. Here's what I narrowed it down to. It appears that one of the pages in my watchlist is breaking the JavaScript in my watchlist, which sounds crazy but it appears to be the case. I have a second test account, which has an empty watchlist. I watched a few pages, and Enhanced Recent Changes works fine there (with about five articles in the watchlist). I then copied my own watchlist to the test account. Then the JavaScript breaks on the watchlist. It also does not appear to be skin-related because I tried Vector and it breaks there too. Gary King (talk · scripts) 19:03, 29 October 2012 (UTC)
It seems that any page that I have resolved feedback using the Article Feedback Tool (it's the thing where new editor can comment on the quality of an article, with a green smiley face, a red sad face, etc.), if I remove those pages from my watchlist, then my watchlist JavaScript is fixed. Gary King (talk · scripts) 19:15, 29 October 2012 (UTC)
Okay I'm pretty sure I figured this out. This one page that I'm watching has unmoderated comments (comments that are awaiting an action, such as hiding it). Since this is the case, Wikipedia usually gives a message saying something like "one of the pages you are watching have unmoderated comments". It seems that this message, as of very recently (six hours ago or so), breaks the Watchlist's JavaScript. I know this because moderating all comments or removing the page from my watchlist fixes the Watchlist. I've already sent an email to hopefully the right person with regards to the AFT. Gary King (talk · scripts) 19:24, 29 October 2012 (UTC)
Version: unspecified
Severity: major