In about the year 2007, a colleague had made a nifty spreadsheet that was password protected because he didn't want anyone to know how he'd done certain 'cool' effects. He had hidden columns and all sorts of stuff like that.
I renamed it as a .txt file and opened it in Notepad (which took a while). Near the start of the file was some text that looked very nuch like it was a password, it was his name plus some significant text. Sure enough, that was the password. I do hope the Excel password protection is more secure nowadays.