Starting in 2003 I spent several years as a principal developer developing mobile office software.

When we launched our excel compatible spreadsheet we diligently formalized every function and wrote tests to verify behavior. It was beautiful.

Then we started receiving lots of bug reports from users because our calculations often didn’t match what they were seeing in Excel. Older Excel versions had lots of bugs and Microsoft had been carrying them forward because at that point it was “better” to carry the errors forward and display results people expected rather than fixing them and having everyone confused by the changed results.

So we copied their behavior as best we could. I wish I had the list still but it was long and full of weird edge cases which caused formulas to be incorrect.