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.