When you are on page X and click on "watch this page" or "unwatch this page", the 1.3.1 MediaWiki returns to the Main_Page.
However, as already implemented in eg. the User_Login procedure, I prefer to be returned to last visited page, i.e. page X.
In module Article.php in function watch() change the last call $wgOut->returnToMain(false); to $wgOut->returnToMain(true,$link);
Tom