If you use Excel 2007 (Currently my company has not upgraded yet), you might be interested in reading the thread below from the google user groups:

Microsoft Excel 2007 Calculation Bug

It looks like a problem with the floating point representation of certain numbers. Fortunately, from what I can see this specific problem is unique to 2007, so if you haven’t upgraded yet you should be ok. Otherwise, if you use 2007 I’d read up on it to see if this affects you in any way.