If you haven’t already heard about it, (very unlikely since it has been all over the news) there is a bug in Microsoft’s Excel 2007 that display the wrong result when doing a multiplication. The bug first shown up with the multiplication of 77.1 * 850 but the Excel team blog tells us that there are 12 specific calculations that are subject to this flaw.
Basically, this is a problem with the internal representation of numbers in Excel that gets translated for displaying. It’s a conversion bug in the UI part so it won’t affect any calculation, charting or other number analysis on those numbers. VBA script and COM Interop are also immune to this bug because they use the internal representation and not the number actually rendered on the Excel interface. Joel of joelonsoftware.com has a really good article that explains the specifics of this bug.
Every forum and blog posts that I've seen out there, with the exception of Joel’s one, shouts that this is a major/critical bug and start bashing on Microsoft for their “gross incompetence”. My humble opinion on this is that those guys are going totally overboard and see critical problems when there are, in fact, none. Since the bug we’re talking about affect only the number displaying but not the internal representation it will affect people’s life only if they do those exact calculations and don’t use the calculation result to do anything else. The chances of that happening out there exists, I agree. However, they are so slim that this bug should not be considered critical at all. Microsoft will probably release a hotfix in the next days/weeks and it will all be history soon.
UPDATE: The Excel team has released a hotfix to correct this issue. The fix can be found here.