Excel doesn't support dates before 1900-01-01, and it incorrectly treats the year 1900 as a leap year[1]. This bug-as-a-feature started in the first Excel for the sake of compatibility with Lotus files.
EDIT: Apparently Wikipedia has a list of "quirks". I haven't vetted any of the citations, but the page claims issues with statistical function accuracy, failures with modulo operations, confusing use of numeric precisions, and an inability to open two identically-named files
[1] http://support.microsoft.com/kb/214326/en-us
EDIT: Apparently Wikipedia has a list of "quirks". I haven't vetted any of the citations, but the page claims issues with statistical function accuracy, failures with modulo operations, confusing use of numeric precisions, and an inability to open two identically-named files