Version: http://www.mediawiki.org/wiki/Special:Version
Go to http://www.mediawiki.org/wiki/Special:Preferences#preftab-5 (watchlist tab) and change Days to show in watchlist: to 10. Try Save.
The result will report "There are problems with some of your input" on the first tab, but there is no problem on this tab. Carefully opening each tab may find the problem (there is a max of 7 on the watchlist days), but this behaviour is highly confusing. Either a hint about what the error may be should be displayed on the first tab (including the tab name), or the tab opened directly (difficult perhaps if more than one tab is affected).
Version: unspecified
Severity: normal