Undo and Redo
The Undo and Redo commands allow you to walk through the history of changes to the current document, up to the point when the file was opened. Before any significant data-modification action, the application saves a snapshot of the document so that you can undo your actions. After executing an undo, the redo command is available to move forward through the history.
The downside to unlimited undo actions is that it can take up a lot of disk space. For example, if you have a 100 MB file and you apply five different filters to the entire file, the undo system will have to save 500 MB of data in addition to the original file. To clear the undo system, close and save the file, and reopen it.