This tip relates to the technique of trapping and handling errors in a formula. Prior to Excel 2007 (e.g. Excel 2003, 2002, 2000), this was accomplished using a combination of the IF and ISERROR functions to test for an error in a formula, e.g. =IF(ISERROR(MyFormula),ReturnSomethingElse,MyFormula). This meant having to write your formula twice (think of your longest formula!).
In Excel 2007, trapping and handling errors in a formula was made easier with the introduction of the IFERROR function, which removed the need to specify your formula twice, e.g. =IFERROR(MyFormula,ReturnSomethingElse).