Gotcha: Before you begin using manual calculation mode, you need to understand a dangerous situation. The calculation mode is global for all workbooks that are currently open. Say that you open WorkbookA and it is in manual calculation mode. You then open Workbook2 through Workbook9, change a few cells and save them. All the while, WorkbookA remained open in the background. This will change the calculation mode on Workbook2 through Workbook9 to manual. While it is easy to see in Figure 303 that something is wrong, it is not easy to notice that manual calculation mode is on in most workbooks. You can see how manual calculation mode can insidiously spread through your workbooks like a virus.
Gotcha: Before you go back to Automatic calculation mode, ask the person who created the worksheet why it is in Manual calculation mode. Sometimes you will find a spreadsheet with tens of thousands of calculations that takes 30–45 seconds to calculate. It is very frustrating when the system pauses for 45 seconds after every single data entry. If you have a lot of data entry to do, a standard strategy is to use Manual calculation mode because in this mode, you can make several changes and then press F9 to calculate.