The Power—and Vulnerability—of the Undo (Control+Z) Tool

Q: I was working in Excel and suddenly realized that I made a mistake when I entered a formula about 10 minutes earlier and that was screwing up a lot of what I did since then. So I turned to the old faithful Undo command (Ctrl+Z), and although it took me back quite a ways, it stopped before I got to the place where I committed the error. Is my Undo tool broken, or did I do something to disengage it?

A: Indeed, you may have inadvertently cleared out the Undo stack where that data are stored. Excel 2003 automatically saves the last 16 actions in RAM. In Excel 2007, that default has increased to 100. But be aware that if you save a file (Ctrl+S) in Excel 2003, all the actions taken before the save are wiped out. In addition, if you’ve selected the AutoSave option, all saves will be cleared each time it automatically triggers a save action—and that’s another reason I suggest you shouldn’t rely on the AutoSave option. (For more on the limitations of using AutoSave, see “Beware: The Save AutoRecover info option does not save permanently,” May 09, page 78). In Excel 2007, Microsoft wisely rewrote the software so it does not erase the Undo stack after a save. Be aware, too, that when you run a Visual Basic for Applications macro, Excel allocates no memory for undoing actions.

If you wish, you can increase or decrease the number of Undo saves by making a small change in the Registry, but Microsoft recommends a limit of 100. This Microsoft Knowledge Base article explains how to do it: support.microsoft.com/?kbid=211922.