00:00 Be careful how you delete cells (where you get Shift Up or Shift left option)
00:27 The cell you moved will show an error message (Good)
00:38 Deleting cells affects all formula below or to the right (High risk as you may not notice it)
Have you ever done something and suddenly one or a few (not all cells) have a REF error. The REF error cells need to be fixed, but watch out for the cells to the right or below as well.