It's not exactly programming, but it's definitally in the same league as the semi colon, and then I'm talking about "," and "." as devider in decimal numbers. Depending on the language settings of the computer (and Excel's) this changes.
And it sucks than when I'm working with big data files, recalculating everything, get an entirely wrong result... and it's always the "." or ",". I hate it. >B
(Thank god for "Find and Replace").