Well, what a about detailed study? Quoting from abstract:

"Microsoft’s continuing inability to correctly fix errors is discussed. No statistical procedure in Excel should be used until Microsoft documents that the procedure is correct; it is not safe to assume that Microsoft Excel’s statistical procedures give the correct answer. Persons who wish to conduct statistical analyses should use some other package."

On the accuracy of statistical procedures in Microsoft Excel 2007, 2010 http://www.pages.drexel.edu/~bdm25/excel2007.pdf http://homepages.ulb.ac.be/~gmelard/rech/gmelard_csda23.pdf