Last modified: 2013-08-24 08:00:44 UTC
I noticed this on the English Wikipedia, which is running MediaWiki version 1.22wmf12 (bbaecbc). While attempting to use the five-pointed star icon to add a page to my watch list, I often see the message: An error occurred while changing your watchlist settings for "[the page title]". I've discovered that if I reload the page, the page can be watch-listed successfully. The error never appears when I've just opened the page within a few minutes, but it appears consistently when the page has been open for a long time. I haven't worked out just how long that is, but it's more than ten minutes and less than two days. If the underlying reason isn't known, it would be helpful to suggest the work-around of reloading the page.
Which browser do you use? When this happens it would be interesting to know if there is also any output in the JavaScript error console of your browser.
I'm using Firefox 17 on Linux. I restarted it, so I can't reproduce the error just now.
There's no Javascript error.
I'm guessing the error condition might be an expired edit token.