Excel is insane, the undo button is universal, meaning if you are editing multiple spreadsheets, you can’t just undo changes to one of them if you made a mistake between editing, you have to undo all changes to all open spreadsheets to get back to your desired undo point.
Joel Spolsky used to work on Excel and he has written some brilliant stuff about it (at least, in its early days). My personal favourite is the revelation that the Excel team had their own C compiler, which kind of takes Not Invented Here into the stratosphere.
