I used to use a password that ended in !! before I knew about the command, before I even seriously used Linux. It caused problems setting up MySQL and even caused issues for a domain host while transferring a domain of mine. I don't use that password anymore :)
But using things that break other things from time to time is probably still a good idea to find software defects. I used to have a user on my machine whose name (and thus profile folder) contained a non-Latin-1 letter and a space.
