During the whole OOXML debacle, there were many examples discovered of ambiguous or flat-out wrong behavior in various versions of Excel over the years, the kind of wrong behavior that could easily have altered the outcome of financial transactions that were managed using spreadsheets. You can probably find some if you hunt for them on Groklaw. I seem to remember one related to a date basis parameter, so that might be a good first keyword set to try.

Excel only adds up to about 6 decimals. I worked on a project of an Endowment Fund (billions) and Excel was flat out wrong when computing billions over decades.

The worst part is: Excel is the Dogma. If your results do not match with Excel, make it.

1 + 1 = 3

Not a maths error, only a display error.

I was guessing that's what pstephens was referring to, but he hasn't elaborated.