I once used a small DNS hosting provider where I used a password ending in '!!'. I was in the process of attempting to transfer the domain to another host but it was taking them days, and they communicated several times they were having odd issues specifically with my account. In that same time I was learning how to use Linux and MySQL, and ran into issues using the same password for the MySQL admin during a command line install. That's when I learned !! is short for "run last command". Took me a few more days to think maybe that's what was causing issues at the host provider, so I changed it, told them what I did, and asked to please try the transfer again, and it worked. I never got confirmation the password had anything to do with it, but I let support know my suspicions.