Edit > Undo / Repeat

Whenever you make mistakes you can recover from this by selecting (Edit > Undo).

This feature is extremely useful and is presentation specific.
This allows you to undo up to the last 16 consecutive actions.
Every time you use the Undo feature, those actions that you undo are placed in the Redo category.
This can be useful as you may accidentally undo more actions than necessary.

SS - Typical things to undo

Undo remembers up to 20 of your most recent actions

You can cancel or undo your most recent action by selecting (Edit > Undo) or by clicking the undo button on the Standard toolbar.
You can view the most recent actions by selecting the arrow next to the Undo button.
SS

Click the action you want to undo
When you undo an action you will also undo all the actions listed above it. If you change your mind you can always redo.

The Undo feature can be used in 2 ways either to quickly reverse your last action or to select from a list of your last 16 actions.
You can repeat the undo by using the shortcut key (Ctrl + Z).
The black arrow on its right gives you access to the drop-down list of the latest actions that can be undone.

Using Redo

The Redo is structures exactly the same as the Undo, occupying an adjacent position on the toolbar.

You have the ability to Redo up to 16 of the most recent actions
The Redo action cancels the undo command. The drop-down list contains the most recent actions that can be cancelled.

Using Repeat

It is possible to repeat your last action (assuming that it was not an Undo).
You can also repeat your last action by using the shortcut key (F4) or (Ctrl + Y).
It is not possible to repeat any of the actions from the Format AutoShapes dialog box.

Things you cannot Undo

Some actions especially those involving worksheet manipulation cannot be undone (such as inserting or deleting of worksheets).
You also cannot undo settings from the (Tools > Options) dialog box.
Saving or Printing

Change your Options

You can change the maximum number of undos from the (Tools > Options)(Edit tab, Maximum number of undos).

Important

The (Edit > Undo) command is workbook (not worksheet) specific.
It is possible to change the registry entry if you want to be able to undo more than 16 actions.