I was just reminded, again, how Microsoft Excel has the worst possible Undo implementation ever.
Instead of keeping track of undo actions separately for each file, it tracks them for all open files together. When you undo actions in one spreadsheet, you will be undoing actions in other spreadsheets you have open, too.
This is the stupidest possible design I can think of. It leads to data loss for anyone who relies on a reasonable undo feature when managing their documents. It is positively dangerous to have multiple Excel spreadsheets open simultaneously, since undoing in one document may lead to data loss in others.
Whoever designed this should be fired from software development, and made to flip burgers instead. There's no excuse for this idiocy in a software product like Office.