Important and useful features/capabilities of a system that are available only if you know the secret code. Examples:
In MS Excel (at least through '98), F4 changes a cell reference from relative to absolute, cycling though all four possibilities. I have never found a menu entry or tool bar icon to invoke this action, and finding the process in the Help file is difficult. (As I recall, it is listed only under the topic that describes the actions of all function keys.)
To see just the most recent change to a wiki page, click on the date of that change at the bottom of the page or click on the time since the change as reported in QuickChanges. A neat and useful feature but hardly an intuitive user interface, or one that is documented in an obvious place. Both link to QuickDiff.
To see the links to a page, click on the title. Everybody knows about this, but it's not at all intuitive, and it took me ages to discover. -- TomAnderson
To see the last changes made on a wiki page, click on the date at the bottom of the page
Also - RightClick? on page to see what browser features are available for actions on the page.