While the regular undo system will undo common functions and operations that are performed in EasySpreadsheet, it does not undo every single possible function or operation. If you need a FailSafe Undo System, then turn the FailSafe Undo System ON. (Note: The system is turned on if Edit -> Perform FailSafe Undo's is checked off.)
The advantage of the FailSafe Undo System is that it creates a FULL copy of your spreadsheet, each time you enter something into a cell. Regardless of what functions or operations you perform, the FailSafe Undo System will record a faithful image of your spreadsheet as you move from cell to cell. When you press Ctrl+Z, the previous image of your spreadsheet is simply retrieved and loaded into memory. If you press Ctrl+Z again, the image previous to this one, is simply retrieved and loaded into memory. Similarly, you can redo steps that you have previously undone by pressing Ctrl+Y.
The disadvantage of the FailSafe Undo System is that it slows down the operation of Easy Spreadsheet. A delay will occur as you go from cell to cell. If you have a small spreadsheet and a fast computer, this delay may not matter - but it will if your spreadsheet is large and your computer is slow.
The FailSafe Undo System is a useful feature to use when you are working on a complicated portion of your spreadsheet, but should be turned off when it is no longer needed.