You just spent two hours making changes to a presentation that you are making for a colleague at the office. You think it looks perfect, but when your colleague reviews it, he preferred the previous version. One problem... you already saved the changes to the file, and forgot to do a manual backup beforehand!

Fortunately for you, you enabled the "Back up previous version" feature in Keynote. Shwew! With this feature turned on, Keynote automatically saves a backup copy of the file you are working on, in case you ever need to revert to it.

The backup copy is saved in the same directory as the original file, and is called "Backup of <original>" (where <original> is the name of the original file).

To turn this feature on, open Keynote, and then open the Preferences (or press Command+,). In the "General" section, check the box titled "Back up previous version", as shown below:

One thing to note about this feature is that it only saves one previous revision. If you save your file 3 times, your original file will be the third revision, and the backup copy will contain the second revision. The first revision is gone forever.

This feature works exactly the same for Pages, and you can also enable it (separately) in the Pages Preferences.