I was shocked to find what might be a calculation error that showed up on my Excel spreadsheet. I am running Mac OS X 10.5.8 on a MacBook PRo and using Excel 12.2.3 and I would appreciate knowing if anyone else can duplicate this. I have two cells named 'aa' and 'ff' and I have another cell where I have placed the formula '=-ff^2+aa'. When the value of aa is 4 and the value of ff is 1, Excel gives me '5' as the result. When I change the formula to '=aa-ff^2', it gives me the correct answer of 3. What's going on here?? It's hard to believe that Excel would be prone to such a basic error after all this time!