About a month ago, I got tired of waiting NeoOffice's ridiculously long load times for networked files, and switched to Microsoft Office 2008. Office 2008 and its Windows counterpart, Office 2007, use Microsoft's new OOXML file formats, at the expense of backwards compatibility (owners of older versions of Office have to install non-obvious plugins in order to open documents created in the new format). Microsoft describes OOXML as "an open, royalty-free file format specification [that] maximizes interoperability in a heterogeneous environment."
Excel 2007 and 2008 include many new functions, including COUNTIFS. While at work, I created a column of cells that used the function, but was dismayed to see an error, since I was following the function's documentation to the letter. I was even more dismayed for it to display perfectly fine on a colleague's computer running Excel 2007. Even more strangely, it displayed normally in Excel 2008 once it had been saved in Excel 2007.