Thimbleby, Harold2008-05-202008-05-201991-06-01http://hdl.handle.net/1880/46532Being able to undo any operation improves user interfaces, for various reasons. It gives the user a greater sense of safety and encourages active exploration of a system. Undo enables users to extract themselves from error states that might otherwise have trapped them. However, undo is only useful if both: the user knows they have made a mistake, and that the mistake can in fact be undone. Clearly, a poorly implemented undo may compromise the user. This brief paper gives an example taken from the Macintosh user interface where a 'correctly' implemented undo-last-action (the standard Macintosh undo) fails the first condition given above because of the clipboard. We infer lessons about the observability of user actions from this example, and look forward to a safer, more generalised interface that takes the concept of generic actions (like undo, cut, copy, paste) further. This paper discusses the Macintosh user interface (Apple, 87); a discussion of its strive for consistency can be found in Tognazzini (89).EngComputer Scienceunknown1991-437-2110.11575/PRISM/31291