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Because Applescript uses the system's event queue, some actions done with Applescript are reversable. For example, if you use a script to move a file, you can check the Edit menu in the Finder and you'll see "Undo Move of 1 Item."
This works in OS X Tiger, not sure if it works before that.
May not work in all applications, depending on what you've asked Applescript to do. For example, if you use a script to enter text in TextEdit, it doesn't recognize the addition as "undo-able" because it wasn't typed.