A few years back I turned up to a party wearing a bow tie and shorts. Someone took a photo of this fashion nightmare and posted it on Facebook. Clearly this mistake cannot be undone but errors you make with your Mac can be reversed quickly and easily. Provided you are quick.
Typing
Imagine you are typing an email or a Word document and you delete a word or a paragraph by mistake. Before doing anything else (this is important) press Command+Z and the text will reappear.
Dragging a file to to Trash
Press Command+Z and the application, file or folder will pop back right where it was prior to its trip to the Trash.
Deleting iCal events
Command+Z and the event will appear back where it was. Or if you have moved it it will move back to its former date and time.
But the Undo command only remember your last two mistakes (usually just one mistake) so its very important that you act quickly if you want to use the Undo command.
