Red-faced Microsoft officials say a patch released last week to fix security vulnerabilities in Excel 2003 causes the spreadsheet to calculate incorrectly.

The error shows up if a patched version of Excel is linked to a real-time data source through macros built with Visual Basic for Applications, according to Microsoft.

The patch, known as MS08-014, was released for Excel 2003 Service Packs 2 and 3.

Microsoft program manager Tim Rains said on a blog post Wednesday that the company has issued a new patch -- to patch last week's patch. "The security update caused a calculation error in Microsoft Excel 2003," Rains conceded.

"If you're not running Excel 2003, this release doesn't apply to you and you don't need to take any action," said Rains.

The original patch was designed to plug a security hole that could allow remote code execution if users open Excel in certain ways -- possibly allowing an attacker to gain control over affected systems.

It's not the first time Excel has flunked math.

Microsoft last year was forced to offer a fix for a problem that caused Excel 2007 to produce significant multiplication errors.

Excel developers said that flaw occurred during calculations that would ordinarily result in, or be close to, the number 65,535. Instead, Excel 2007 would produce a result of 100,000.

Microsoft's Excel problems come at a time when its Office franchise is under threat from free and low-cost alternatives offered by well-heeled rivals like Google and IBM. IBM's free Lotus Symphony suite -- which includes an Excel-like spreadsheet -- has garnered more than 150,000 downloads since its release last year.