If a user open an old revision of a page in edit mode and then click on [Show changes], it will be displayed a diff against the most recent version of the page and not against the version he was editing (sometimes it is this diff which the user wants).
Such a feature is available in [[User:Js/ajaxPreview#Extra_features]], and is very useful.
See also bug 29674.
Version: unspecified
Severity: normal