Some of the worst ones:
* Microsoft Excel will not open two documents with the same name and instead will display an error
* Excel includes January 0, 1900 and February 29, 1900, incorrectly treating 1900 as a leap year. The bug originated from Lotus 1-2-3, and was purposely implemented in Excel for the purpose of bug compatibility.
* Can't handle dates before 1900
* Despite the use of 15-figure precision, Excel can display many more figures (up to thirty) upon user request. But the displayed figures are not those actually used in its computations, and so, for example, the difference of two numbers may differ from the difference of their displayed values.
In addition, it is completely useless for handling any type of delimited file. By default it supports csv. If you want to edit a pipe-delimited file, you have to dig into the language settings of Windows, change the default delimiter, and open the file. Then, you cannot edit comma-separated files without changing it back first. So, there's no way to convert one to the other. Same problem with tab delimiters. EDIT: one person has mentioned the import data feature, but that doesn't solve the problem. You cannot edit and save anything other than commas without changing the Windows language settings.
It also trims off leading zeros from fields in csv files. If you open a csv file with leading zeros in some rows (say, from ZIP codes or SSNs), they won't be displayed in Excel. Then, when you save it, even without editing it, all the leading zeros will be gone.
It changes date formats. Save a csv file with a date in it in the format YYYY-MM-DD. Open in Excel: it's displayed in MM/DD/YYYY. Hit Ctrl-S. Re-open the file in a text editor and you'll see that the file has changed.
It also doesn't preserve quotes correctly in csvs.
Besides all this, there are many other stupid terrible things about it, like changing behavior based on scroll lock (can you think of one other program that does this?), making it impossible to look at many tall rows using the mouse scroll wheel, etc.
Excel is easily one of Microsoft's most egregious offenses in the world of software.