If you check items on Special:EditWatchlist in 1.18+ that are not on your watchlist, submitting will result in a "There are problems with some of your input" error, with "The value you specified is not a valid option." at the bottom of the offending section. As a result the watchlist will not save and the user has to submit the form again for it to work. This error would usually (always?) happen because the user clicked the link to page and unwatched it from there. Since the form is intended to remove pages from one's watchlist, it should not be considered an error if a page has already been removed. It should submit successfully, like it did in 1.17 and earlier.