formulas pulling data from other worksheets
 

In what situations would it be necessary to press F2 on a cell to get it to
refresh/calculate and allow any changes in the cell it's pulling from to come
through.

Example:
Worksheet 1 Cell A1: =worksheet2 cell a1

If the information in worksheet2 cell a1 changes, and the workbook is set to
calculate automatically, why is it sometimes necessary to place the cursor on
worksheet 1 A1 and press F2 to get the new result to come through.